we've bought my daughter 2 phones from ebay. we have alltel and the alltel rep told us just to get a number (don't remember which one it was) by the battery and tell the alltel company. they run that number to make sure there is nothing owed from that phone. if that's clear then when you buy it and get it in the mail, We went to the alltel store and they activated it. Had good luck with 2 phones at alot cheaper price.
We just emailed the person and asked for that number before we purchased that phone. But i have noticed that some people are posting that number, so you don't have to ask for it. My other daughter bought a razor off of ebay this past spring. worked good til she broke it....

that was her fault though. hope this makes you feel more comfortable.