Your comments

I've been on various past and current sites where you list books, cds, dvds, etc. where the media is exchanges directly from person to person. The site/ platform only handles the listings, you enter the tracking with them, and the credit only gets released when delivery is confirmed ( by tracking + 10 days past delivery without issue reported or by the end user confirming receipt without issue... whichever comes first. ) One of the oldest which is still going strong is PaperbackSwap. They have been pretty transparent and well documented over the past 20 years so looking to sites like that might be useful for precedent.

On the business end, a number of these sites/ platforms deduct a small fee from each exchange ( and have fee-free exchanges as a perk of paid membership. ) Since Pricecharting already has collector and retailer subscriptions, I imagine that would probably integrate well.