How to buy a phone from ebay?

I wanted to know how do I buy a phone from ebay. I have metro pcs and was planning to buy a phone but did not want to pay the full price at the store. I wanted to know what type of question should I ask the seller before buying the phone? How can I tell if it is stolen? And after I bought it if all I have to do is go to the store and tell them I have a new phone? Thanks for help.