I would simply state that I prefer to play by ebay's rules. And if the seller's intent is to save on fees, I would offer to have them cancel the sale. I would leave no feedback and walk away.
There will always be more crap to buy, it's not worth to be upset by it.
Heck I'm not upset. It was a complete Stronglight P3 headset which I could really use for a Mercier 300 I'm rebuilding but no big deal. The P3 has a stack height of 34 mm which is lower than any other French threaded headset I can get my hands on (Velo orange is 41; I have a Campy NR french threaded headset but the stack height is 41).
I can figure out a different way to get that bike rolling again.
I've offered to cancel the sale and I'm waiting to hear from the seller. I suspect the seller will let me out. If not, I'll have eBay deal with it.
I do think it's a little flaky for the seller to try to operate outside of the eBay payment system and to try to get me to buy the item paypal friends and family especially for an international sale.