A lot of people are saying to take the ID, but isn't checking the ID sufficient? Besides, there are other things, like a bank statement, which confirm the name and on top of that their address.

I guess if you live close to the buyer you could ask to meet them at theirs. You see them leaving their house, how exactly are they going to hide from you?

Another idea is to pick a good spot. A tennis court for example would be naturally closed off, stand next to the only gate and voila, they'd have to use force to get away with the bike. 

Also, sometimes just the good-old first impression is sufficient. You can base the decision of whether you will let them try it out on that.