The guaranteed price would have to be extremely low, like when you take your big box of old DVDs down to those retail stores that buy them back used for $0.50-$1.00 each.

I'm not sure if this service is hitting what I see as one of the major pain points of selling your used electronics on cragislist or eBay: You have to list each item separately if you want to get any kind of reasonable price. Who else has one of those "big piles of old computer junk" stuffed in their closet? I do. There are probably a few items in there I could still get $40 or $50 for individually, but if I just photo the whole pile and sell it, I won't get more than $50 for everything.

That's an interesting question, but I think the problem of bootstrapping enough users pales in comparison to dealing with fraud. As an aside, I read that Amazon itself buys items from their used marketplace if it believes they are underpriced.