Iphone 4 help on ebay!!!?

I want to buy a iphone 4 on ebay, it says it is locked but is jail broken. The iphone is with AT&T and I have AT&T already. Will I be able to just put my sim card in it and it will work? Will I be charged? I have a LG xenon right now is there a difference in sim size? PLEASE HELP ASAP! I have to decide if Im gonna buy it or not! THANK YOU!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!