As a potential user, here are a few things that would cause me some concern:
- Will there be someone there who has an idem I actually want, in return for the items I want to swap?
You either need to have a LOT of people signed up right from the get-go, or you might need to "salt" the swaps at first in order to get the first swaps kicked off and let your advertising/word of mouth carry it from there.
- What security is there that once I've sent my side of the swap away, I'll actually receive the other item?
Maybe you have both send their items to a central warehouse where they get matched up and then forwarded on to the correct recipient?
- (Related to the previous point) What is my assurance the item I'm swapping for is in as good a condition as the other party says?
I don't want to swap my like-new iPhone for a pair of Manolo Blahniks that were advertised as "only worn once", only to discover that the "one wearing" was apparently on a cross-country hike through ankle-deep mud.
I'm not saying it won't work! In the world of books/music/DVDs you've got services like BookMooch, Bookins and Paperbackswap which show people may be willing to swap, as long as their are some protections/assurances in place.