For it to not be theft you need an agreement which says he may sell it.
But imagine this scenario: I don't know you, but I dump a pile of stuff in your backyard. A lot of stuff, with some relatively valuable stuff in it, like copper.
You get home and what? Are you expected by the law to put your sweat and/or money to get rid of that stuff from your backyard?
Or is it perfectly legal to call somebody which will clean that up for you in exchange for the value of the stuff? They might even profit from that (after all that's profit in exchange for work)