How do you prove it’s stolen? More importantly how do you expect the business to figure out it’s stolen before they purchase? People bring in legitimately acquired scrap metal, including copper, all the time. I knew guys growing up that had that as their primary source of income, buy/ find junked cars and other random metals (usually in impromptu “dump sites” that happen on the edge of towns), take them apart, sell the metal. How does a business tell the difference between them and a guy dissecting houses under construction? That’s part of why thieves are into this stuff, no serial numbers, once they safely get away from the theft site copper is copper.
That being said, if someone comes in with fresh copper or aluminum coils with company stickers still attached (as happened to one of the companies I worked-for as a result of theft), you can't look the other way. You're receiving stolen property.