Hey, find a medicine man and you can actually get some good trade for those.
Apparently even today still, IIRC leaches actually do get used for legitimate medicinal purposes in the modern day. Them and maggots, I believe for cleaning out necrotic flesh.
You find three goblins in a trenchcoat with a shack full of dubiously acquired merchandise of poor quality. They'll buy your loot for 12 silver, a shiny button, and this really cool bug they found yesterday.
Sure be a shame if all that selling screwed up the economy enough to completely offset any gains of collecting everything they can carry and immediately sell it instead of, y'know, roleplaying how actual people in that world would behave.
I think my next campaign needs a pawn shop that'll consider buying anything but will seriously low-ball it if they can't move it easily. And maybe need to talk to an expert they know.