Hello Ejlif,
I am unsure of this, as I've never personally taken this route, because luckily, every transaction I've ever had here on Audiogon have been absolutely flawless, regardless if I was the Seller, or the Buyer of goods.
So far, I have been very happy, and 100% satisfied with my membership here, with both the classifieds, and also the priviledge of being a participating member of the forums as well.
What I believe makes Audiogon a good provided service, and resource for it's members, is that most here choose to do business in an honorable, and truthful manner.
With that being said, I feel you as a buyer are definitely entitled to the terms of service that Audiogon provides for its Members, and without adherence to such guidlines, and terms, would we then allow unscrupulous business transactions, thus duping the unsuspecting to be allowed?
I am unsure of this, but there may be no need over concern about the issuance of a negative feedback being given by you, or the worry of a retaliatory feedback from a less than honorable seller.
I once years ago had little choice on ebay to give a negative feedback, and thus got retalitory from the low life seller, over a Marantz 2330B Reciever, listed as mint, and basically was a basket case instead, and since ebay's largest concern was they got thier money from the seller for listing his/her Auction Listing, I got the only neg in all my feedback over the years. I rarely if ever use ebay anymore because of this, bacause ebay has little interest in who is right, or wrong.
The seller though lost his account, due to many negative feedbacks, and then I understand from one buyer, threatened the buyer with sending him a virus on his computer.
If I were you, I would carefully read the Dipute terms, and guidlines to fully understand what your rights are under Audiogon Agreement. The seller may have very well not noticed the flaws of this Cartridge, but from what you have written sounds to me more like he's denied that there was any damage, and claims it was YOU who damaged the Cartridge. This to me, is grounds for a dispute process to take place.
Don't feel like you are "The Bad Guy" for initiating such a process, that's what it's here for, and IMO neglecting this process, you have in effect let Audiogon turn into another ebay. Mark