First off don't worry as long as they have been honest with you that it was feeding prior to you getting him then it will be fine til he starts up with you.
Second what was it being fed before you got him it needs to be the same prey item til it acclimates.
Third do not handle him or her til its fed a few times with you just dont mess with him at all.
http://ball-pythons.net/forums/showt...s%29-Caresheet
The above link is to our caresheet read up a little maybe it can be of some help.