I joined Swap.com a few days ago, and found out a little too late how their exact methods work, and it's not something in which I feel comfortable participating. I don't like the method they use for acceptance and rejection, because it puts you under the spotlight to get the item out now now now. I mean, I can't make two or three trips to our post office every day. That eats up a lot of fuel. Add in Swap.com's monthly fee, transaction fee and shipping costs, and you're really not saving any money at all!
I can't recommend them as a way to really save money. I'm sure there are much better choices out there.