Have been a long time Ebay seller as well; 98% of the time sales go smoothly. Have sold over a hundred slabs and have been fortunate so far. The biggest scam sellers have to face is when a buyer receives your item, they can claim you sent them either:
a) an empty box, or
b) something entirely different from what was in the listing.
So they open a Not As Described return case and demand a full refund. They can't return your item anyway, because after all, you didn't send it according to them Or they'll send you back junk in a box. If you refuse the return, they cry to Ebay and they force the refund anyway. So now the buyer gets their money back AND they steal your item. And it doesn't matter what evidence you have in your favor, Ebay's not gonna care. It's bad enough when it happens over an item under $100, but I've heard horror stories of high-priced item thefts, where the only option a seller has is to get the local police involved and hope that spooks the scammer into sending the item back/dropping the scam. It's one of the most infuriating problems out there, because you know you've gotten screwed, and Ebay will just be like 'meh'. They made a decision a long time ago to kiss the buyer's behinds.
It's actually safer to sell via Ebay's International Program now, because the rules are different and have extra protections for sellers. Buyers won't be able to get away with the same scam, because if a buyer wants to return an item for whatever reason, and you have No Returns on your listing, the buyer has it returned to an Ebay hub, where Ebay refunds them out of their own pocket (so Ebay eats the cost, up to $2500 I believe), and then Ebay will liquidate/sell off the item in some manner. You as a seller get to keep the proceeds of the sale.
Selling on these boards is sounding more desirable. If there's ever an issue between buyer and seller, CGC isn't going to play Referee like Ebay does.