Considering buying a second-hand Apple TV 3rd Gen.

I would not bother really

3g do not have an app store, only some predefined server side channels where most of them has

been discontinued, and post here which could suggest changes in airplay mean 3g is not very supported anymore

and some content apps on ios devices block airplay of their material.

Even Netflix will end their service for this model by July 31st.

A pre-owned Apple TV box, that wasn’t restricted through enterprise device profiles, can be reset to factory settings, so that the new owner can use it “as new”. That would remove any account information, settings, and other customizations, so that it can be set up again and re-registered. Reset or restore your Apple TV to its factory settings - Apple Support

If the device was restricted through an enterprise device profile, then that profile has to be removed by them first, else it isn’t fully accessible for anyone.

Apple may also sell refurbished products, that were returned, then checked to be found in good condition. Certified refurbished products - Apple Store online (US)

Other resellers may also sell Apple refurbished products, but without Apple certification. So, not the same quality standards per se.

What I meant to inquire about is the 4K 3rd Gen 2022 model.

Those are the current and latest models [A2737 and A2843]. Identify Apple TV models - Apple Support. Not sure why anyone would want to part with it already, but if you can find it for sale pre-owned, then that might be a good option.