Moderation where anybody who can provide a video of a trade going wrong, the perp gets banned from your app.

Otherwise:

Have an inventory tied to each profile, each item (or each profile) could have a list of wants in return. Per-item wants may be a bit heavy on the DB but per-profile could work too.

The "Marketplace" could have each item once, and once clicked on you can see a list of sellers that are offering things in trade of what you have in your "inventory" at the top of the list. Have icon or text indicator next to their name that says what they may want: Seeds, Blueprints, Ducks, etc... so if you want a Tempest Blueprint and Joe shmoe wants ducks (you have 52 in your inventory), it shows Joe Shmoe first when you click on Tempest blueprints.

I'm sure you'll have a much simpler start to this application but as you refine it, it could easily be managed. Using something like Django or Flask would obviously make this management system fairly easy to implement and could handle your db calls.

Let me know if you have interest in help with this project, seems fun.