Most of these concerns are the same as with any site that deals in blind transactions (ebay, used things on amazon, paypal in general), but these sites only require you to risk your time upfront where as in the case of this site the customer is risking something they already own with the hope of future payment. Essentially its like requiring people on ebay to send items before they receive payment.

The most basic way I can think of to deal with this imbalance is to have the shipper make a deposit up front to "insure" the item being ships. Upon arrival the shipper gets their deposit plus payment. There is probably a more elegant way to frame this so that the shipper does not feel like they are risking so much.