A serious one
Charity shops, car boot sales and Amazon's second-hand book market.

Buy the books dirt cheap, sell them on Amazon. Sometimes the mark-ups are ridiculous.

The best books to go for are degree-level textbooks. A lot of ex-students (or more likely their parents) dump their old, bulky textbooks on charity shops or flog them at car boot sales. Now charity shops hate these books, because they are almost impossible to sell (who the hell goes to a charity shop to buy their uni textbooks on the off-chance they'll have the one you need? No-one, that's who.). Thus these books are available for next to nothing (most uni texbooks cost £20 or more, and they're no more than £2 or £3 in a charity shop), and you can snap them up.

The books you will get are usually a couple of years out of date, so avoid subjects that change frequently (law, politics, compsci) and stick to subjects with subject matter that stays pretty much the same (philosophy, history, anatomy, literature). Even better, get a hands-free kit and have someone on the other end by a PC to check the ISBN numbers and their second-hand value.

My record was 3 Philosophy textbooks bought for a combined £2 at a car boot sale, later sold for a combined total of £41.

Kerching.
(Axeman Jim, Mon 4 Aug 2008, 14:21, 3 replies)

now this
is a good idea

I go into charity shops a lot for books, so I will be keeping an eye out
( Vipros. clever got me this far, then tricky got me in, Mon 4 Aug 2008, 16:31, closed)

Me too!
And I live in a city which has THREE UNIVERSITIES!
( chickenlady b3ta's very own Elizabeth Taylor, Mon 4 Aug 2008, 17:08, closed)

Checking
It's a lot easier to check prices and such if you've got a smartphone or something else with Web capability. I've done that with my iPhone plenty of times.
(FleetlordVT, Mon 4 Aug 2008, 20:46, closed)