I only buy off ebay now as it's become a bit expansive to sell stuff.
If I really want something, I'll wait till about 8 seconds to go before I bid. I'm a sad computer nerd (used to be a programmer) that will sit with 2 windows open, one with the bid ready to be sent and one being refreshed. BUT, like all other auctions, don't bid over your maximum price and don't forget the p&p.
Check out some of the stuff from Europe and the US too. Even with the shipping and exchange rate you can get a bargain.
I've also used it to find obscure things that I wouldn't have a clue where to find a shop that sold them (rubber feet for an ironing board, don't ask!!!!!).