If i get a att iphone 4s can i switch it to simple mobile?

I need to know so i can buy a iphone from ebay and put the sim card from simple mobile

Yes you can! But make sure it is unlocked and if not you can get vendor on eBay to do it or like 15 bucks.

Yes. It needs to be unlocked. Simple Mobile uses T-Mobile. You will be limited to 2G data and MMS likely won't work either.

First of all, the phone must be unlocked before you can use it on SimpleMobile. Secondly, the phone won't have 3G on SimpleMobile as the 3G bands of SimpleMobile is not compatible with the 3G bands of iPhone 4S. I would suggest you to opt for StraightTalk if you want better service.