Appraise check | EN World Tabletop RPG News & Reviews

If you want middle ground, you can cut their profits at sell time. Tell them rough value of the gems as written in module (so they can note it down), but when they actually sell it, roll for streetwise/whatever and modify the amount of gold they get for it.

This skill rolls would combine both their skill at selling/haggling and retroactively real worth of the hoard - in any proportions you want. As nice side effect, you could apply modifiers to the roll depending on the size of city, gem supply etc.

One important thing - after they roll, they sell it. No 'tries'. If they just want to appraise, give them original value, of you really want to complicate things, compute the value in given settlement assuming they have rolled 10 on streetwise.

Now, actual rules for interpreting skill result into percentage of value...