If they get RMA'd because they are over the manufacturers limit of dead pixels then they should only be re-sold as such, i.e. seconds.
If on the other hand they are returned because of the distance selling act, i.e. the number of dead pixels is below the manufacturers limit but not acceptable to the user then I suppose there is no reason why the vendor cannot re-sell it as new as long as the packaging etc. doesn't prompt the next buyer to suspect anything.