How do I switch Phones without switching plans with Sprint?

Here is the case

I quite literally watched a man throw his old (but still working and usable) Samsung Instinct into the trash can

This guy is a prick and doesn’t want me to have his old phone cause he doesn’t like seeing people have things as nice as him…

I went and grabbed the phone when he left…

I want to switch from using my Samsung Reclaim and start using this lightly used Samsung Instinct

How do I go about doing this…is it illegal of me to take it, when I bring it in…will they try to return it to him…or because he deactivated it, is it free game????

Thanks for all the help…not sure what to do