People should realize these are names bought for SEO purposes, not as brands. They are for reinforcing an existing brand. It would be like Nike.com buying "runnersforsale.com" and redirecting to the mothership or developing another site.
Mattresstopper.com sold for $21500 to an end-user, most likely. Reseller value of that domain is $2000. I would therefore put your domain at 10% of that to reseller. So low $xxx to reseller.