I've bought two Macs (a Quicksilver and a single-processor G5), a couple of monitors, my trusty HP Laserjet and all manner of other items on eBay. Other than a first disaster with a Powerbook, which I've detailed at length many times before, no problems. (scam artists took the person from whom I was buying the machine - I did not deal directly with them myself). I'm very careful though - on high-priced stuff I never deal with someone with no feedback, very little feedback, or someone with a less than near-perfect record. If there is any negative feedback, I read it to try to get a feel for what happened.
I've managed to get very good deals - but that's only after exhaustive research on prices and specs etc. Two things you can watch for in particular - Buy-It-Now auctions that are priced low (look in the newest listings - they go fast) and items that have a too-high starting price. Sometimes that's enough to discourage anyone from bidding, even though if you actually bought it for at or near the starting price, you'd have a very good deal. Wait, watch, be patient. Don't bid on anything until the auction is almost over (unless it's a Buy-It-Now with an attractive price - in which case you want to snap it up before someone else gets to it) Bid in the last two minutes and bid with an odd amount (ie: something like $423.67 rather than $420) and bid the maximum you're willing to spend. Then hit refresh to find out if anyone is trying to out-snipe you!
Of course, if you've truly made your top proxy bid, at this point you're simply watching to find out if you get the item and for how much! Remember - even if you place a proxy bid of $10.12 on an item sitting at $3, the bid increment and any other proxy bids will determine what you actually pay. If nobody else has made a proxy bid, you could get it for $4. If the top proxy bid from another bidder is $10, then your $10.12 bid will win, unless someone else gets in there and ups the ante. It's hard to do that in the last two minutes though - which is why you want to put in your top bid and be done with it. At least that's my philosophy....