I would say once you change anything with it you cannot expect a refund, even if you make the razor better. I think this is fair to both parties.
If the item wasn't as described you should get a satisfactory resolution with the seller.

As long as there isn't deep rust at the edge I would keep it. The only reliable way to get a pristine blade is to pay the cost of new one. On ebay unless the item is described as new I wouldn't expect it to be excellent cosmetically, even though it can happen. The black rust doesn't spread by itself if the razor is dried and stored properly.

Your theory seems good to me - the razor probably has just one shave on it but wasn't stored too well. In any case I'd give the seller the benefit of doubt on this one as there's no strong evidence to support the contrary. If the seller described the cosmetic condition wrongly I would talk with them about it and either return it for a full refund, keep it for a partial, or if I feel it's worth the price I paid I'll just keep it but still tell the seller his description was incorrect.
I think you should decide how much this is worth to you and if less than $200 you ask the seller for partial refund or send it back for full.