I buy scopes off eBay and various classified sites and the best bargains are those scopes which have bad ring marks.
I'm not talking crushing or denting tubes but normal to severe ring marks such as you'd find on scopes tightened by Bubba.
My wife uses these foam pads sold as Magic Erasers from Mr. Clean. I tried these with a bit of water and a bit of dishsoap and with a little elbow grease even bad ring marks virtually disappear.
I was amazed how well these worked.
Link to Mr. Clean Magic Eraser.
http://www.mrclean.com/en_US/magic-eraser.doThere are other brands out there as well and they work too.