It's a nice idea, but I would not pay $100 for a 'chance' to win something like this (maybe $15), especially when what I am paying isn't going to charity. I would rather just flat out buy them if I wanted Speedster humps. I also think you may have trouble finding that many people who want black speedster humps.
But by all means try it out, and if it works it could be the next eBay.
