Upload & Sell: Off
Sorry to hear, is the serial number in the description or pictures of the sellers ad?
I'd send pictures and a 3rd party's evaluation of the lenses to both the auction house and the seller asking/demanding they remove the bad feedback.
I might also add that if nothing is done that your last act in this matter will be to warn the net not to use their service. Then to plaster the forums and review sites.
No serial numbers nor close-ups of the lenses. I did submit a question to the auction house support asking about the unjustified feedback, and they basically didn't want anything to do with it and told me to try and settle it with the seller. Also, they will not remove feedback unless it is explicitly malicious or if requested by the feedback provider (which seems stupid and short-sighted on their part, but does cut down the need for personnel).
Oh well, I think I'll just let it be. No point in trying to start a one-man-war against the site, as it's the largest (and pretty much only one) in Finland.
Besides, the Sigma does look salvageable after all - there was a little fungus only on the back lens, and it cleaned off well. The back of the front lens does look a bit hazy, though, so I'll probably try to take it apart for cleaning at some point. Still waiting for my Konica adapter.