Can you buy an AT&T iPhone 5s off of eBay and use it with Verizon?

I know the iPhone 5s can be used on any of the main North American carriers but I know each carrier has their own locks and stuff. If I bought an AT&T iPhone 5s off of eBay would I be able to just put in my current Verizon iPhone 5's SIM card into it and it would work? Or are there steps I have to do first? I know it's possible, I just need to know the steps in order to do it.

AT&T does not use iPhone models with CDMA radios. A CDMA radio is required to run on Verizon's network. It will not work in any way.

It can be done. If it is locked to AT&T it would have to be unlocked. Only AT&T can properly and legally do it. If it's contract is up they will do it without charge. If it is still under contract there will be a early termination fee. Be very careful about buying phones off ebay or used in general. Get the serial or IMEI and call the carrier (AT&T) or Apple to see its status.

I'm VERY leery about ebay phone. People lie all the time and many people like to simply sell a problem phone with outstanding payments.

If they phone is on the up and up contact (AT&T) and ask for an unlock request. They send an unlock code to the phone. After 24 hours restore the phone using iTunes on a computer. Then go to Verizon and get a service plan setup or simply put the phone on an existing plan. This usually requires a trip to a physical Verizon store.

You can't use AT&T iPhone 5S on Verizon. Verizon is CDMA only and doesn't support GSM. AT&T iPhone 5S is GSM only and will not work on Verizon. Verizon doesn't use a SIM card so they're incompatible. If you want to use the phone on Verizon, buy one that's on Verizon. If you need Verizon iPhone 5S without contract cheaper than retail, mail me.