I've bought over 500 items over the last 16 years, and only one or two gone bad.
Just keep your wits about you, be very wary of where the seller is situated (China can be risky I've found, especially if you want to return the item), don't get "caught up" in an auction by going above your decided price, do a bit of looking round to see if prices are realistic. There are some sellers coming on to eBay now (especially in the Collectors category) with ridiculously inflated prices because occasionally they will get a mug paying it. Sometimes there is a reason for above average prices so you might need expert advice (take a look at £2 coins if you want to see an enormous range of prices, some of which I can't believe are justified, but I'm no expert).
Anyway, good luck with your eBay trading.