I'm not sure where payment fits in but I read a while back that craigslist uses http://www.reducefraud.com http://www.reducefraud.com to validate users via a te

I'm not sure where payment fits in but I read a while back that craigslist uses http://www.reducefraud.com http://www.reducefraud.com to validate users via a telephone call. Google App Engine signup does something similar with SMS messaging. Would a telephone number work for what you are doing?