How do I know, if I buy a cell phone on ebay, that I will be able to get it activated?

Well Im not an expert on this, but I do know sim cards are used by AT&T and T-Mobile, might be one other carrier that does to. These two carriers usaully require that you sign a 2-4 yr contract throughout which you pay a hefty bill, I believe also a Credit Check is required as well. I would look into carriers such as Cricket, Boost Mobile, Sprint etc. You might find their services a bit more helpful. If Im incorrect I apologize, but hopefully it will help some small way. Good Luck ^^