Like most other people say, he most likely just wants a copy of ruby, but if you really want to do it, first:
1) Check his feedback, and if he if he has at least %99.5 check the items. (Even then you should be very, very careful.)
2) Try to get to know the person a bit to see if you can trust him. Don't do it straight away.
3) If it looks the slightest bit dodgy, DON'T PROCEED WITH THE TRANSACTION!