are my iPads unlocked - need to know before I sell them

I just purchased a new iPad and I am cleaning out old devices. I have returned two iPads - one an iPad 3rd Generation and one an iPad Mini 3 - to factory settings and have removed the SIM cards. They were sold as ATT iPads but there is no contract associated with either iPad and, as I mentioned, the SIM cards have been removed.

MY QUESTION: Can I sell them as "unlocked?" That is, are they compatible with all cellular carriers? I don't want to sell them as unlocked if they are not truly compatible with all carriers.

Apple, of course, won't even answer the question because my Apple Care Protection Plan has expired for the devices.

Pls reply ASAP! I need to get these up on eBay soon because of personal scheduling.