Sellers know up front that if they start an auction at a low price and without a reserve, they may not get what they hoped for. I am often amazed when I see Mitch's listings for really nice chips starting at $0.99. Like, almost always. I asked him about it and his answer was simply that he gets more attention starting out low and doesn't end up losing much value over time. Nevertheless, I've picked up a few chips from him for that $0.99 starting price that had book values of $20 and more. He never complains and just sends the chips. Then again, I've bid pretty high on some of his other chips and lost as 2nd highest bidder, so he can thank me for pushing the winning buyer up by at least one bid increment (and probably more).

Your guy did a buy-it-now and you beat everyone else to the punch. Sour grapes from the other buyers, so too bad. They weren't contacting him BEFORE you bought them to let the guy know they'd be willing to pay more, now, did they? As for whether their contact is ethical, eBay doesn't preclude people from doing that (I think). I've reached out to sellers of closed auctions to see if they have another available for sale before. You could try and file a complaint that the seller is trying to extort you for more money after the fact, but he may just decide to "lose" the item, refund you, and sell to one of the other guys for more money, so tread carefully.

If it were me, I would just stop communicating with any back and forth about value, price, etc. Just send a "Need tracking number" message periodically but ignore irrelevant responses. If he doesn't comply (send you your items) within a couple of days after paying him, contact eBay support for help. If he has a great rating, he won't want to jeopardize it, especially if you give negative feedback and mention the extortion attempt.

Good luck!