If I buy an iPhone 4S on eBay, would att (my carrier) let me use the iPhone? - 1

What would I have to do to make it were was my simcard would be in phone. Where would I have to go to?

If it's unlocked or an AT&T phone then it would work provided it's not marked lost or stolen.

The 4S has a SIM tray on the side that you open with a paper clip or something similarly small to insert a Micro SIM. You can sometimes cut a regular SIM to a micro.

Yep, as long as it's unlocked you're fine. Just go to the ATT store to get a new sim card.

I have sold every model of iPhone from the very first one. Now they were all working on AT&T's network and that's how I listed them when I sold each one of them on eBay. So this is what you need to look for in the sellers listing.

When we (a seller) list phone(s) to sell there's an option of what carrier the phone is tied to. Most reputable sellers will list that but if the phone is listed as unlocked you technically are able to use the phone with other carriers.

Now I'm not an expert on unlocked phones and all the carriers they will work on, but my best advise is find an iPhone that is already on the AT&T's network. You could unlock the phone yourself. There are codes and services that will do that for you. The choice is up to you but I would keep it simple.