I've bought two used BlackBerry devices off ebay. They came with two different scenarios. Both were simple to deal with but even if the phone is as described, sellers always seem to forget something (I know I do ... heh).
First one, the 8700c, when I called to set up my data plan and everything, his IMEI and PIN hadn't been released but it was a two-minute fix by the rep. I was up and running pretty quickly.
Second one, a red Pearl I got two weeks ago, the person had left all of their personal info on the thing. I had no trouble switching to it online when I logged into the BIS. But, her contacts, photos, all that stuff were still on there.
Maybe a little info on wiping a device so you can start fresh?
All three of the used BB's I've purchased were from sellers with excellent ratings, feedback and longevity of membership (on ebay or here, in one case). They also provided multiple actual photos of the phones for sale.
There's only one ebay seller I trust that uses stock photos and the store doesn't sell BB's.
Ebay is a great place to find a deal. I managed to get a LNIB red Pearl for $135 shipped. :D
You just have to know where and how to look.