Other answers (including my own comment above) address possible reasons your players are doing this, and possible solutions to mitigate it. But you're not asking for value judgements or how to stop their behavior, so here's my best shot at a neutral analysis of how a D&D type setting could respond to the behavior.
These are some responses I've used, or considered using. They aren't intended to be used all at once, and some are much harsher than others. Players engaging in this kind of action are calling attention to themselves, and the world will respond accordingly, but as Flamma points out punishing players is a bad idea. These are possible reactions, intended to make players more aware of the consequences of their actions, create complications, or give the world a bit more depth. Consider these springboards for your own ideas, and always consider what you're teaching the players about your game and your world before you do something drastic or mean.
First, a little context
Depending on the setting this behavior isn't quite as shocking as you're making it out; a gold coin in most D&D editions is worth 10 silver, not 100. Still, the denomination is unusual and even if a place of business makes hundreds of gold a day they're unlikely to see a lot of gold coins go through the till.
"I can't take that; nobody here can make change for it."
Sure, the party isn't expecting the local innkeep to make change for a platinum piece... but nobody else can either. It's like trying to pay someone with a barrel of crude oil; sure it's worth a lot, but what is Bob the Fishmonger going to do with it?
Same applies to objets d'art, handfuls of gems, and rare/ancient/foreign coins. Finding someone who can evaluate and buy their stuff could be a fun RP adventure, and there are all kinds of stories you can tell around an auction house.
Lavish them with inconvenient praise
Your players are clearly disgustingly rich philanthropists with no sense of proportion. Swamp them with beggars, inundate them with charity representatives, give them tedious award ceremonies to attend in their own honor. If they can't maintain the level of charity to which the locals become accustomed, turn the crowds against them, for reasons the next point makes clear.
This is largely going to be local color, but if the party wants to engage then it give them some toothy moral dilemmas.
The economy booms and crashes
First everyone runs out to spend their newfound wealth. It spreads around, people get excited, luxury items become popular, new businesses open, people move into the suddenly prosperous town... but it's all supported by itinerant charity.
Soon after the party stops their lavish spending or move away, the town can no longer afford its new luxurious tastes. Businesses close, investments are lost, unemployment rises. With an increased population the town can't return to its old status quo: no longer able to afford imported food and having grown faster than its farms could plant and harvest more crops, the larger population begins to starve.
This could provide a lot of new hook: helping revitalize the town could go a lot of different ways, or maybe the villain sweeps in to save them and gets a townfull of people who really don't like the PCs.
Give 'em lackeys
Sherlock Holmes' Baker Street Irregulars come to mind: when someone tips well, he attracts a lot of willing employees. I'd be surprised if some enterprising young urchin didn't pop up to be an eager errand boy. Bonus points if he's got some connection to a plot.
Induct 'em
A high-profile group with plenty of money offers just the kind of publicity and profit many organizations are looking for. Adventuring clubs, philanthropic societies, shady organizations, all would love to have the party as members. Of course, there's an admission fee. And membership dues. And obligations to be fulfilled. But it's very prestigious and members get discounted services at participating locations!
Seriously though, membership can provide access to exclusive quests and resources, an information network, accountants and lawyers to handle the legal issues (see some of the point below), and so forth.
Mug 'em
Clearly these newcomers are rich and naive. Every mugger, pickpocket, cat burglar, confidence man and used car salesman in town is going to see a golden (sorry) opportunity. The party's going to be beset by thieves, get-rich-quick scammers, and people trying to sell them a London Bridge / Eiffel Tower boxed set.
This can provide a lot of interesting action/RP, and maybe even a new adventure: a renowned cat burglar comes to town to steal from the PCs, and the PCs are hired to catch him?
Refuse the coin, call the cops
What's the modern reaction to paying for a stick of gum with a $100 bill? You check if it's counterfeit. If a group of people make a habit of this, the law is going to start making inquiries.
This can go a lot of ways: uncover corruption, make a friend in law enforcement, flee and get a tracker or bounty hunter on their tail.
Note: this is why you see people biting coins in old films. Your teeth scratch off the paint on a fake and show the dark lead underneath. [There's a common misconception that you bite the coin to test its softness, because gold is softer than most other metals. This is incorrect; pure gold coins would wear down just rattling against each other in your pocket, so the gold is always alloyed with another metal to make it harder. If anything, a lead coin (lead was used in counterfeiting because it's about the same weight as gold) would be softer.]
Bring in the bureaucrats
Once the PCs are found to not be counterfeiters, swindlers, or thieves (not necessarily an easy thing to prove, given where most adventurers get their wealth), send in the tax collector and the customs agent. Moving undeclared wealth over borders? Dodging taxes through border hopping or simply not paying? Oh, you say you have no permanent place of residence? Then we'll have to collect your full dues and fines right now, and don't worry; your place of residence for the next six months is the local jail.
Bureaucrats are the nuclear option unless the party likes this kind of thing. Use with caution.
"Those counterfeit coins aren't mine! I got them off this guy I killed... I mean... um."
Seriously, what are the chances that all that junk the party finds in orc pockets is legit? Maybe the cops or the taxmen find something suspicious after all. Or maybe just explaining how they got their wealth is enough to get them in trouble.
See "call the cops" above, but with the added potential for uncovering a new evil plot for the party to stop based on the junk they've been hauling around.
"So that's where they're hiding."
Regardless of how the local population responds, chances are that the party becomes very high-profile very quickly. Is anyone looking for the party? Say... maybe a recurring villain, or someone who wants to set them up as patsies, or a person from a PC's shady past, or someone with a quest for them to undertake?
This is a carte blanche excuse to bring in anyone you want. Be creative: bringing back minor but popular NPCs in a larger role is a favorite of mine.
Best Answer
Start by asking your players why they're only interested in material rewards. If this group of players is not terribly interested in RP or story, and mostly want to play a game that allows them to collect money and XP, then you won't get very far trying to introduce story goals. On the other hand, it may be that your players feel that unless they ask for a reward up front, they do not get anything out of the adventure (which may mean that you as DM should drop more treasure/XP/whatever in general).
If they profess to want story goals, then start by adding goals that directly impact one or more PCs. For example, instead of poisoning some random farmer's daughter, use the fighter's younger sister, or the wizard's niece. Or even have a PC himself be poisoned. Basically, make it clear that the penalty for failure will be more than "nameless one-shot NPC dies".
Then, once the players have agreed to take on such a quest, find a way to reward them anyway. When they go looking for the mushrooms, perhaps they find the skeleton of another adventurer (and all his treasure) who had been poisoned like the girl, who had died trying to get the mushrooms. Then, later during a different adventure, they come across someone else who's been poisoned, and they can sell the location of the cave or any leftover mushroom stock.
This eases the players into the idea that not all adventures will have explicit this-for-that rewards. Once they accept the idea that not all rewards will be stated up front, but that treasure will still be dropped at interesting and exciting points during the quest, your players will be more willing to take on quests that don't involve direct material rewards.