I have bought one from ebay a while back and never got any problem. Of course, make sure to read the description as sometimes the picture doesn't correspond to what they are gonna sell you.
& Feel free to ask as many details as possible with the seller as well, reguarding if the one they are selling correspond to your needs.
I'm not sure what the prices are nowadays, but Dell labtops have always been working
amazingly with me, and I never experienced
any problem with the last one I had!
Is your girlfriend looking for a plain labtop to do daily stuff on it, or is she going to play games with it?