They give new set, the testing set is separate on the display on the shelf.
I can imagine all the user marks on the lenses, bodies, and lens mount. Not forgetting the burning of shutter counts. Don't think I'll want a so-used set. Tried to test at TK Foto the other day for the 16-35, was rejected, citing that testing is only allowed if there's the intention to purchase, else the unit will be too used to be sold after multiple tries. Fair enough, I was going to buy anyway, so I just informed them. Not many shops these days prohibit testing and allow customers to molest their stock all over. Well used sets can always find the mount full of use marks and/or dirt/scratches. Like what Catchlights says, we know, we go in, we buy and leave. You try going back to the shops within 7 days everyday ask about the lens performance, is it normal, etc etc and see if there are any after-sales service.