Ebay is way overpriced and 99% crap. No bargains there. Plus, the only time I used it--about three years ago--I was trying to buy an army canteen for my kid and the seller ripped me off. There is no recourse, of course. Some redneck criminal named Travis (true!) from Arkansas. Hate ebay.
I have loads of used and indy bookstores near me, including one of the biggest and most famous, Rodger's Book Barn, which is on a dirt road in the middle of the woods and only shelves quality stock.
Plus, there are loads of thrift stores, and each has a bookcase full of used books. These are especially good for 10-year-old hardcover bestsellers.
BTW, I am really surprised at the prices charged for used books in UK. On a recent trip it looked like the going rate for a used paperback was 3-4 pounds, which is what new costs in US, and they were really tattered.