Yeah, I'm pretty leery of it. I'm all for them signing a pre-sale agreement saying "you break it you bought it", but having them just sign something means nothing if they get on it and don't come back. Honestly I don't think asking for a deposit is too much to ask; if they're not actually interested in buying it, then they don't need to be riding it anyway. If they return it unscathed and they don't want it they get their money back and go on their way. I'm also not just concerned about losing the bike, but I also wonder what my liability is if they hurt themselves, whether they sign a liability waiver or not. People are just too sue happy these days.
Better living through chemistry.