Hi Whateve!
I too have returned countless fakes to the sellers who ask for them back, but I do ask them to pay for the shipping. And like you, I have also had very nice sellers who did not want them back and refunded me anyway, or replaced with something else.
Funny thing is, just yesterday I bought a group of four bags on ebay and every single one was a fake!!
You might wonder how I was so stupid to not see they were fakes from the pics, well it was only one photo, and it was a $20 BUY IT NOW sale plus $10 shipping. I knew that the little LV in the pic was going to be fake, and maybe even one more, but I would still be getting a bargain for the two "real" ones. (And I would NOT have made the seller issue a partial refund because I bought it knowing at least one was fake). But I was shocked to find them all phony...lol. The seller sent me a full refund.
Now if only I knew what to do with the mountain of crappola I have accumulated......