For me, it all depends on the situation...The way I see it is if from one of my many thrift stores that I pick up from, its mine and what ever I do with it once its in my truck is my business. They alrighty had the chance to sell it for more rather than to me. Btw, most of the thrift stores I pick up from I pay for the stuff. I get the metal for cheap tho. Like $50 a truck and trailerload. Example. I bought a load back in July for $60. Well in that load was a HUGE professional flat bed scanner, cost like $3k new. Brought it home had wifey check it out and throw it up on ebay. Sold for $400 as is untested.
If its a foreclosure or one of the rentals of the many property management company's I work for, well I don't even worry about it because if they wanted it then they should have taken it....
If its a property clean up then I always ask.