I decided I want to buy an "unlocked" cell off eBay. A lot of phones listed say they work with "any GSM carrier". Some are T-Mobile phones but say they will work with AT&T.
My question is....is this true?

Some of the T-Mobile phones are cheaper on eBay, so I'm wondering if I put my AT&T sim card in, will it work or will the sim card read as invalid? :dunno:

Help me out!