How to unlock an AT&T iphone?

So I'm planning on buying an AT&T iphone 4s from ebay. It is not unlocked at the moment, but from what I understand if the contract is up AT&T will happily unlock it for me. But since I'm not the original owner, I don't know if the contract IS up or not. Is there a way for them to check their databases or something? I need to unlock it so that it can be used with Straight Talk, if that detail makes any difference.

I've also heard that you can purchase the unlock code, but I'm a bit wary of that. Is this a good alternative to having AT&T unlock it, and if so can you recommend where to buy the unlock code?

Thanks for reading, any and all advice is appreciated!

Glad you're looking to unlock the iphone, it raises the value of your phone and gives you more freedom.
The status of whether your contract is up or not does not matter because you will be switching out the sim card. You just have to make sure that the past owner of the phone does not report your phone as stolen(you can check how to see if it's reported stolen/blacklisted from the link in the sourcebox)

And unlocking your iphone should be no struggle at all. I've used information of this one site(sourcebox) many times to flip iphone and gain extra cash. After reading the information and you thinking it may be shady, remember that eBay is a protected buying service and if there are any scams on eBay, they will eliminate them immediately so you won't have to worry about that.lol, you'll be surprised how easy it is to unlock an iphone now a days compared to the past