It's true that StraightTalk offers both CDMA and GSM service, however, so far as the iPhone is concerned, they are not a supported carrier for GSM iPhones. They are only supported as a CDMA carrier, meaning you would need a phone sold specifically for use on StraightTalk. you may be able to make it work, however, there are lots of reports of being unable to get Data and MMS messaging working among other things. You would have to work with StraightTalk to resolve those issues if it's possible.
Working with a carrier (MVNO) owned by Wal-Mart, I don't see a lot of joy coming out of that.
In addition, if the phone you're looking at isn't already unlocked, I wouldn't touch it. If you're not a customer, AT&T is under no obligation to unlock it for you.