A similar thing happened to me last year.
I got mine refinished in Metacol III (black) by Arizona Response Systems (
http://www.arizonaresponsesystems.com ). The gun looks great and the grind mark is not as noticeable any more. The grind mark used to bother me so much that I actively disliked the gun; now it doesn't bother me at all.
Try to work it out with the seller. Call H&K, confirm that the gun is a refurb with no warranty, and get the name and number of the H&K rep you are speaking to. Also, find some "comps" on the internet to show what your refurb is really worth. Clearly lay out your position in a cordial e-mail to the seller, inviting him to call the H&K rep and check the comps. If you want to return the gun, say so; if you want a partial refund, propose a reasonable figure. Give the seller a reasonable but definite deadline to respond, and if you don't get satisfaction, then call your credit card company and tell them you want to dispute the charge. If it gets to this point, you will be able to show a good faith effort to resolve the dispute, and you'll have all or your reasons laid out in a copy of your e-mail to the seller.
The bottom line is that the gun was represented to be new when it wasn't. When all is said and done, you should get some relief.
Even if the seller may be a crook, give him an out by assuming he made an honest mistake. If you back him into a corner by accusing him of ripping you off, it will make everything more difficult.
Good luck!