In my setting money is fairly important, as I am running a West Marches style campaign. I don't want the one lowly trader in the village to be able to purchase everything the PCs wish to sell, but I also don't want him to be completely penniless.
I'm assuming traders don't have unlimited cash, and would have a rough amount depending on the size of their cart/market stall/shop/emporium.
How much, or how do I determine how much, the average "trader" would have on them in order to buy PC goods?
The DMG gives wide latitude to the DM to determine how the PCs can sell their stuff, and so it does not give any numbers or tables for estimating the amount of gold a particular trader has.
Instead, it takes a different approach, where it asks the PCs to try to find a buyer for magic items (DMG 130):
For each item a character wishes to sell, the player rolls ... The subsequent total determines what a buyer offers to pay for the item. You determine a buyer's identity. Buyers sometimes procure rare and very rare items through proxies to ensure that their identities remain unknown. If the buyer is shady, it's up to you whether the sale creates legal complications for the party later.
While this is specific to magic items, the DMG doesn't have much else to say about the wealth of merchants. As you can see, this advice is mostly geared toward making new plot hooks, rather than crafting a reasonable economy.
I've never liked putting limits on player wealth by what they can sell. In the best case it doesn't matter at all, and in the worst case the PCs end up with a ton of "valuable" crap they can't get rid of because of an arbitrary-seeming cap.
Instead, I've just assumed that most treasures/non-magic/non-plot items they find are automatically converted to gold the next time they visit town. If I want the PCs to have limited wealth, then I limit the treasure they can find. If I want them to be forced to go somewhere else to sell their goods, I give them items that the merchants don't want to buy, or have them search out buyers, as described above. This is a lot more efficient, I find, than doing this kind of bait-and-switch trick.
There are no printed guidelines for this
I have previously given merchants about one-quarter of the value of everything they have for sale. For the sake of avoiding what I would deem unnecessary role play, I've never had a merchant unable to afford something a PC was trying to sell, but rather used this figure in the event the PCs murder/loot the merchant, and thus far 1/4th the value of the wares has felt appropriate and doesn't seem to be broken.
