I've thought about this a bit for homebrewed beer (illegal to sell, but perfectly legal to give away and/or trade). My only thought is that you could manage the shipping of the items--have each trader send you the item, where in you inspect it, and once you've received both, you send both along, otherwise, you return the original widget to the sender.
Charge shipping plus a fee for that service. Plus, if you have a centralized shipping location, you'll be better suited to get deals on shipping costs.
This, of course, ignores the fact that it's illegal to ship alcohol via USPS, and it's against policy for UPS and FedEx.