I assume that there are limitations on what kind of loot can be sold in markets of a given class. It doesn’t make sense to me that the PCs could sell really high-value gems or whatnot in any normal village, or even fairly large towns.
But I can’t find anything in the book stating precisely how to deal with this. I expect I just don’t know where to look.
So, help me out: How much and what kind of treasure can PCs turn into liquid assets in any given size of community? I assume that if they have thousands of GP that they want to make portable, I can assume there are gems available according to the guidelines on the chart in the Equipment chapter.
“Mercantile Ventures” (p142) might be useful to you, although it seems to apply to bulk items more than individual items.
I think you can also just reuse the Market Class tables. The chances of finding a buyer being about on par with finding a seller.
Also, I think it was stated somewhere that gems are effectively high-value money and don’t need to be “sold” so much unless you’re just trying to make change.
Ludanto's answer is the same I'd give. In general, if PCs want to sell goods, you can simply reverse the Equipment Availability chart and/or use the Mercentaile rules, depending on the type of good they want to sell.
For example, using the Equipment Availability by Market Class table, if your PCs are in a Class VI town and want to sell a 600gp suit of baroque silver-coated plate armor, they'd have a 5% chance of finding a buyer.
one small clarification: is it reasonable to treat gems as a fairly liquid source of wealth, or is it necessary to convert them to gold? I was under the impression part of their value to adventurers is as an easy means to transport large sums of wealth.