I have bought every BB (2 7290s, 1 8700) I ever had on eBay. Even sell my used equipment on there as well.
I tend to watch for individuals selling one they just got or someone upgrading. Never had a problem, did get one that was never wiped and guy left all his info on BB. I have bought dozens of phones from powers sellers and never had a problem.
Just read the auction from top to bottom, read the feedback. If they have the same type of bad feedback stay away.....where there is smoke there is fire.