Speaking as someone who has tried something like this, maybe D&D isn't your best system.
Here's why, and what needs to happen to get around the problems.
Money is Finite
In each level of D&D, players are expected to earn a fairly exact amount of money, ranging from not much to being able to buy large cities out of pocket change for healing potions. Selling things doesn't produce money, it merely changes where the money gain occurs.
This means a few things.
First, don't worry about maintaining inventory. At all. He's a greasy merchant with a cart. So long as he has the cart, he gets paid. He gets paid whenever he/the party needs money. Instead of getting grants from kings or rewards from grateful villagers... he defrauds the ungrateful villagers from their money.
The horrible truth that you both have to agree on though is: this doesn't actually change how much money would be earned by the party.
What it does do is make a lovely narrative and a huge plot token for adventure of when the cart is stolen. Also, every three levels, you should generally upgrade the scope of the cart, to keep pace with the amounts of money being exchanged. Plan, in mid paragon, to sell a kingdom a monorail.
For normal magical items ... there's no really good answer. Selling common items is silly, and it's impossible to create uncommon magical items. Personally, I would allow the party to respec their items if the bard/party can overcome a skill challenge (as a DM, I just allow players to choose level appropriate magic items every level because it means I only need to worry about plot magic items. This provides a narrative framework from that. The value gained in the respec because of a new level can represent the "truly excellent scams" that the bard comes up with.)
Having "the right item at hand."
Happily, there exist rules for this. If your merchant is human, ask him to take wild talent master as his free feat. (One of my most favourite feats in the game.) Refluff one of the powers, Mental Tools to be representative of him having "just the right tool on hand." And, of course, the fact that they don't last is even more illustrative of the ... quality... of his fine fine goods.
Make sure the rest of the players are invested in this.
This concept is horrible for normal dungeon crawling. If the party is willing to engage in an urban game, perhaps even with the Break & Enter third-party stealth rules, this can be quite fun. As a concept attached to a normal adventuring group, this concept will be just horrible. Here, maximal refluffing is your friend. Have a complete decoupling of powers from their appearance, such that the party can fit into the venue and archetypes allowed by this world. Furthermore, make sure the entire party is cool with where the "who do we defraud" line is drawn. 4e is not a good game to model subtle party conflict nor direct PvP. Take everyone through the Same Page Tool and probably joint character creation before continuing. Finding the right setting is also important.
What I've Found
Using only ki-focus implements has a very limiting effect. They are one of the smaller magic item groups and very few of them have properties or powers that add or change damage types. Since I was looking for being able to permanently deal one type of damage, I've ignored ki-focus implements that only have daily or encounter use damage type changes/additions:
- Fire Blazing Arc Ki-Focus
- Thunder Thunderfist Ki-Focus
- Lightning Forked Lightning Ki-focus
Of those, Thunder had the best feat support. There were damage feats for dealing thunder damage (2) and feat letting me slide an enemy 1 whenever I hit them with thunder damage (reinforcing the controller aspect of the home brew class).
Ultimately my solution was to stick with ki-focus implements because switching to any other implement type would have raised my magic item requirements. Using swords as implements would require having 2 magic weapons kept up to par and all other implements would require 3 magic items total (needing to use both a silvered and non-silvered sword to get the most of my class features).
Here are a few wondrous items that can be helpful:
Bag of HoldingDDI Level 5 Uncommon - Reduces the load by up to 199lbs for endurance weak characters... Though it is level 5, it is given out in a treasure parcel at the end the new Red Box set's module (when the player reaches 2nd level.)
Battle Standard of HealingDDI Level 3 Uncommon - Power (Encounter • Healing, Zone): Standard Action. When you plant the battle standard in your space or an adjacent square, it creates a zone of healing energy in a close burst 5. Whenever you or an ally spends a healing surge while in the zone, you and all allies in the zone regain 1 hit point.
This effect lasts until the end of the encounter or until the battle standard is removed from the ground. Any character in or adjacent to a battle standard’s square can remove it from the ground as a standard action.
Chime of AwakeningDDI Level 4 Uncommon - Power (Daily): Standard Action. You set the chime of awakening to ring when a specific trigger occurs within 10 squares of it. Example triggers include the presence of anyone other than you and your allies, the light of the sun touching the area, or the appearance of a specific character or type of creature. The chime rings in the minds of you and all allies within 10 squares of it. You and affected allies are instantly awoken (if asleep) and alert. The chime of awakening can be fooled by creatures in disguise. It makes active Perception checks with a +10 modifier.