I've bought a few dozen CDs off eBay in the last few months. Too many according to my missus. Anyway, I have generally thought they were OK - the adage 'You get what you pay for' still runs true. There are a few good prog providers who I have used a number of times with no problems. Prices are cheaper than in the shops and generally have, over time, a wider range. I've more or less exhausted the titles I am interested in the high street stores. Some of the prog sellers are casting their net over a wider area which is great.
Ebay is also good for getting what I call my B-list and C-list. What's that? Well I have a theory that it is no good in just having the best in your collection as pretty soon you end up having a worst (of the best) section. Better to have a B and a C list which puts your best (A list) into perspective. On the other hand I am not prepared to pay full price for a B and C list CD so a second hand CD is a good compromise.
The only thing I have about eBay is the postage. I'm not certain as I haven't sold anything yet what goes into the postage costs but the variety of costs does seem a bit strange. Presumably this covers the costs of the ebay advert, postage and packing and PayPal charges. Anything over £1.50 seems a bit excessive to me. But then some suppliers have discounts for multiple sales.
You just need a bit of patience as it could take up to a week after you have won the bid before the package arrives. Some are well worth for though. Try Salva.
You catch me at a weak moment as I haven't got any bids in at the moment