I think the best thing you can do in an eBay transaction is to come onto this board and do a search on the seller--the eBay user name, the name of the closing company, etc., etc.
My first HGVC purchase was from an eBay seller, and I only placed a bid because other TUGgers had posted here that they'd had smooth transactions.
I'm in the midst of a second purchase, and the agent, again, has a good reputation, here.
In each of these transactions, I was okay with bundling the ROFR and the estoppel letter. I got what I paid for in my first transaction, because it was a reputable seller, and in my current transaction, I've read enough feedback around here to know that if something takes a left turn, the agent will fix it.
Reputation is everything. There are a lot of scammers out there, but there are reputable eBay sellers, too. Do your research (and there's nowhere better to get info than right here), and you'll be fine.