I'm starting a new campaign. All fresh players.
In the first session I gave them a very shoddy, small barge, which I planned to have them trade for a ox-drawn carriage, as the ship itself didn't have much life in it. The NPC that was going to trade this to them lowballed and offered 5 platinum pieces (50gp). The players, instead of objecting to this price, accepted. This, of course set them up to fail in the travelling aspect. I do feel bad punishing them for something they did not know of, but I don't want to give them a sweet 16 and have a carriage magically appear.
How do I resolve a situation where players' decisions prevent me from moving forward? How do I prevent situations like this happening in the future?
Best Answer
In the short term, there are many ways to resolve this - have the person selling them the carriage inform them that they've been taken advantage of and advise them to shake down the boat buyer for a bit more, have them take pity and offer the carriage for a lower price, give the PC's a chance to do a small adventure and earn the money, etc.
But there's a bigger issue at stake here: you're not offering the players agency. A reasonable definition of player agency is "the ability to make informed choices that affect what happens." That informed is key - "choose the left passage or the right passage", where the left passage leads to almost-certain death and the right to fabulous treasure, does not count as agency. It also doesn't count if you make whichever passage they pick be the one that leads to danger (the "quantum ogre" effect.) To be clear, either of these "choices" (where either it's essentially random or you're controlling the outcome) can be an occasional part of a good game. However, if you offer the players information (the skeletal remains of previous adventurers crowding the left passage, for instance) then they can make a real choice about how much risk they want to take, because there are some foreseeable consequences, and players generally prefer that.
So how does this apply to your situation? In an unfamiliar world where the players have no idea how much money is worth, they had no reason to believe that 5pp wasn't a fair price for the barge - and no particular reason to care even if they knew, if all they wanted was to be rid of the thing. So now it turns out it was important for them to do something differently, but they had no way of knowing that at the time, which can be pretty frustrating. If your intent was to have them sell the barge so they could afford a carriage, you should have set the situation up this way:
Now your players have enough information to make a real choice - try to talk down the price of the carriage? Steal it? Earn the money somehow? Sell the barge for enough to make up the difference, like you planned? Sell the barge, keep the money, and walk? Ignore your plan completely and take the barge down the river? Each of these has some more or less foreseeable consequences, letting them decide what direction to take the story. Even if your overall story is on rails, you want to let your players fill in some details with their own style; otherwise, it's just a novel, not an RPG.
One other thing worth noting: there's a distinction to be made between player skill and character skill. Unless the characters are completely naive, it would have been totally reasonable to say "By the way, Fergus, it occurs to you that even for a boat in poor condition, that's a pretty low price. Still want to take it?" In games with a lot of newbies, I usually try to set things up to emphasize character skill, so it doesn't feel like I'm punishing people for not knowing the system well yet. An easy way to do this in 5e is the "passive skills" mechanic - anyone with sufficient Insight or Perception or whatever should be able to notice certain things even if not paying special attention.
Player skill can also affect how you provide information to support player agency. Experienced players, you might say, know full well that it's risky to run in a dungeon, and therefore if they're doing that anyway, it counts as an informed decision. New players who aren't as familiar with the mechanics, tropes, and expectations might not realize that cautiously checking for traps and enemies is a possible and prudent thing to do, so you may want to tell them rumors about the deadly machinery in the tomb, or ask them out of character if they're checking for traps. Same goes for expecting your players to remember names and info and take notes vs. reminding them if they seem to have forgotten. Ultimately, the details are a matter of style, but the important thing is that players feel like they know what they're getting into.