I sell on eBay.
First - contact the seller - see what they are going to do? If they agree and do refund your money, make sure to leave feedback once everything is finished. just list the facts, and outcome
if seller ignores you... or won't refund.
Open a case against the seller who sent you the knock off, this is against eBay practice's. This way you can get your money back, and eBay can track someone that is not following the rules, also leave feedback that states you are unhappy, just list the facts, and outcome.
For buyers that gives you some crazy lowball offer. I just laugh and decline. If I counter offer on an item, I give one counteroffer and that's it, if I am not offering free shipping, I make sure to write in the notes, plus cost of shipping listed. I don't go back in forth. If someone offers back and comes within 1.00 or so I will consider it, again checking if someone slipping in free shipping.
I have learned not to take it personal on the best offers. Just hit decline and move on. Really those type of people are just that way, do you really want them as a customer?
For anyone that suggest the " off the records transaction" report them right away. This can get you kicked off eBay. In my opinion this is opening yourself up to bigger problems.