Maybe I missed this somewhere. Just as there are rolls for determining purchase availability, is any similar rule used for unloading gems, jewelry, and art objects recovered from a dungeon? On the one hand, it seems easier for the PCs not to worry about such things, but on the other, it seems implausible that this Class V small town can dish out a few thousand gp for ancient jewelry.
This post has some numbers that may be useful.
http://autarch.co/comment/16179#comment-16179
Most notably; the total value of all market transactions in a market is 100 gp per family. I would say that that is the hard cap of how much money the market can give to the PCs per month.
I believe there’s some posts that are more on-point around here somewhere, but can’t find them at the moment.
For common gems and trade goods I don't usually sweat it. For singular items like high value jewelry pieces I've just used the same market availability as for making a purchase. So if you could buy it on demand you can also find a buyer on demand. It occurs to me I don't know if that's intended to be a symmetrical relationship; quite possibly not, but it has the virtue of being right there in print when I need it.
I do the same thing
For small individual items that aren't too expensive, I use the Availability by Market Class as a guide...I might let the party sell five times as might be available for purchase, but not ten times as many. For more expensive or unique items (high-value gems, jewellry, objets d'art, etc.) I use the Magic Item Transactions by Market Class table on pg. 227 of the ACKS core book. For bulk goods, I use the rules around Mercantile Ventures as a guide...meaning I follow them if it's a significant transaction, otherwise I might only make some of the rolls, and eyeball it.