I've only ever had one problem with buying a second hand deck from eBay - the deck was so musty it smelled like it had been buried in a dank basement for 25 years. I tried all the helpful tips that I've ever seen offered up on various forums, but I think it's forever permeated with this smell. Still, I like the deck and don't want to toss it into the trash...so I store it double wrapped in zip lock freezer baggies so the condition doesn't spread.
I love eBay and have gotten some super fantastic deals there. (not just Tarot) I don't scour the listings constantly, so I'm sure there's a lot of nifty 'Buy it Now' options that get grabbed up before I would have a chance to see them.
I've never had any problem with a seller either - I *always* check the feedback and if there's negative feedback that sounds like a legitimate complaint (rather than some crank customer just blowing up) then I just skip bidding on the item at all.
I've never sold on eBay, but my mom does sometimes and has been pleased with how the transactions went. (however for the sake of clarification, these were non-Tarot items)
Blessings,
Sally Anne