Those too. Basically, if you price the book at 1p, Amazon doesn't get their 15% (?) cut, which is why so many sellers charge 1p (or 1 cent). But if the postage is more than £2.75 (in the UK at least), they have to charge more, and then Amazon get their cut.

Basically, it's hard to make a decent profit by selling used books on Amazon, however, you can just put up the price. I've seen most used prices shoot up in the last few months.