If you're buying anything big, I would buy from somebody who has a feedback rating of at LEAST 10. You have to figure that for every three sales that he successfully completes, one will probably leave feedback - so that's around 30 successful sales, definitely enough to believe the person is trustworthy.
You can get screwed over on eBay, but I NEVER have. Not as a buyer. I have run maybe thirty to fifty auctions, and only a few times have I had a winner who did not follow through on his bid. Sadly, nonpaying bidders always retaliate when you leave negative feedback on their accounts. That's something you have to be wary of.
I've sold lots of big stuff, lots of small stuff, and pretty much always received what I thought the item was worth. Sometimes, I even got more than I thought I would. That's always fun!
As a buyer, it's important to get into an auction knowing how much you want to pay. Most people don't bid until the last 60 minutues of an auction (This tactic is known as "sniping" - fun if you're a seller, not so fun if you're a buyer!), and it's easy to get caught up in the heat of the moment and spend more than you intended.
Plus, eBay is really a seller's market, and items sometimes will go for more than what they're worth. All the same, I've gotten some great deals. One of my favorites is
my Roland MT-32. It's a popular sound module from the late '80s. I think it originally sold for about $600. I don't know what it's current blue book value is - I believe it's about $150-200. I got mine for $30 from eBay. It's in pristine condition, and it came with the original power pack. It didn't come with the manual, but I just downloaded a .pdf version from the Internet and printed it out.
My other favorite deal - and it's actually not music-related - was a classic computer, a Tandy 1000. I had one when I was a little kid back in 1985, and now that I'm older, I'm beginning to collect classic computers and games from the '80s. It's geeky I know, but I love them. For $25 (plus about $25 for shipping), I got all this: The computer WITH memory upgrade to 384K RAM (the memory upgrades cost several hundred dollars back then - my old Tandy actually had 640K, but 384 is enough to play a lot of games), high resolution color monitor in perfect condition with NO burn-in or discoloration, keyboard, joystick, two floppy drives, printer, 300 baud modem/phone, a bunch of other hardware that I can't remember, and a CRAPload of software, much of it in the original packaging. The software includes version 1.000 of Sierra's King's Quest II, and the AGI version of King's Quest IV (only a few thousand copies were made of the AGI version, while about a million copies of the SCI version were made (it required 640K of RAM on the Tandy), so it's highly rare). I won't bore you with the rest of the details, but a few of those items will be worth a great deal of money some day. I've already been offered cash upfront for some of it, but I think it's more fun to play with - and I got it all for a bid of $25.
So yes. If you're willing to look, you can get a STEAL on eBay. I'm a fan.