If the phone has the logo of a phone company on it, like AT&T or T-Mobile, then it probably is NOT unlocked, and can only work with SIMs from that same company.
Some phones can be unlocked to work with SIMs from any company, but I don't know if yours can or not; you would have to call AT&T (or T-Mobile, or whoever's logo is on the phone) and ask them if you can unlock the phone and how.
If they tell you how to unlock the phone and you successfully unlock it, then in your eBay ad you can say it's unlocked. But if you can't unlock it, or you aren't sure if it's locked or not, then in your ad you should say that it is an AT&T phone (or a T-Mobile phone, etc.) and also say that since you never unlocked it, it is locked to only work on AT&T (or T-Mobile, etc.).