I have bought lots of batteries on eBay and most (but not all) have been fine.
I bought a spare battery for my S2 for about £3 and, while it wasn't quite as good as the stock battery, it worked well and lasted the two years I kept the phone.
I have just bought another two spare batteries and a charger also on eBay for the grand sum of £12 and again, they seem to work fine.
You are taking a small risk with eBay batteries, but they are so cheap that it's a risk worth taking.
I do agree with sntaylor though. Spare batteries are a far better way of extending battery life than those horrid extended batteries or battery cases. They are hideous.