I, too, have been on ebay since 1998 and like Russ use a snipe service for some things. My feedback is only 401 which I shall, rather self-servingly, chalk up to a more refined sense of restraint.
In addition to the excellent advice Fnarf and Ross have given I would only add that you be sure you absolutely know what it is you are buying.
I do buy a variety of things on ebay but the big money stuff I buy and sell are motorcycle parts. And virtually all of those are from old BMW airhead motorcycles. I know those bikes pretty well and I know the fair prices for many parts, accessories, goo-gas, etc. I know what to expect.
I also buy some fly fishing gear and antiques. With stuff like that a very slight difference in condition, series, manufacturer, year made etc can mean a big difference in price.
A particular emphasis on caveat emptor is warranted when buying on ebay.
And look into paypal if you haven't already.
Good luck.