Hiya!
Pearl (50gpv), Malachites x2 (25gpve), Gold Frog with Ruby Eyes (150gpv). <-- I just made up those values, I'm not sure if you have actual values assigned for yours or not. If you don't, well, uh, that's how I do it. I just make it up based on how much I figure they would be worth in my campaign world. Now the fun part. PC's trying to sell them. First, they have to be in a position to actually do so. With the above, I'd figure any old jeweler/gemcutter would do...there isn't anything really out there or overly expensive. Now, I have the player make a roll on whatever he has that will help him, or CHA (e.g., if the player says "I'm a dwarf, and I have Appraise as a proficient skill", he can make a CHA check with his characters Appraise skill bonus, with Advantage; if he wasn't a dwarf, no Advantage; if the jeweler/gemcutter was also a dwarf, no Advantage). Next, I would multiply the roll by 5. If the total is 100% or greater, they get the value listed. If it is lower, that is the percentage they get for the total...if they want it. If the PC balks at that (say his roll equaled 12, so 12 x5 gives 60% of the 250gpv total; or, 150gp for the lot). At this point, we roleplay. I role play up reasons for the lower value...."Well, this Malachite has a fault-crack...when it's cut, it will break into several small ones, hardly worth the full 25gpv. The Pearl is ok, but it has discoloring here...and it isn't all that round; Yaniss, a friend of mine from the cost, visits every couple months and brings me lots of pearls that are better quality than this sad little one here. The Frog is cute and all, but, well, I just don't see it selling anytime soon in all honesty. Hmmm...no, sorry; I'll give you 150gp for the lot and not a copper more. What do you say?"
And that's how I do it. If a player rolls REALLY well (like high 20's), and then also roleplays why he should get more for them, and his roleplaying makes it seem advantageous to the buyer, I will let them get more. Not a LOT more, but definitely more (maybe up to about !20% to 150% at an absolute maximum). But in that case, next time the PC's have to deal with the NPC he may make his displeasure known about how he was "ripped off by the fast-talking shysters" and whatnot.
Of course, that all assumes a smallish town with only one jeweler/gemcutter. If they were in a large city, I usually just say "You get the value after spending a full day shopping around". If the PC's only want to be "in town for an hour or two at most"...then we roll, as above. Basically, if the PC's have the time to try and find the best buyer, they can usually get 100% of the value...but it will take at least a full day.
^_^
Paul L. Ming