I've been a Powerseller for 5+ years, but I sell a product that I manufacture and produce so there's plenty of profit. If you want to make a living out of eBay, I do think this is one of the best routes to go. Of course, buying low and selling high is much easier, but with eBay, almost everything already has sellers and competition already so it is tough to find your own niche and do well since the only competitive edge you have would be on selling lower than others.
Manufacturing or producing your own products doesn't have to be as hard as it sounds: printed T-Shirts, drawings, putting jewels on cell phone cases, custom made stickers with a vinyl machine ... I know this sounds like a lot of work (and isn't really BH), but it could potentially be a good niche if you land in one.
You can only find so much old stuff to sell around the house that people will want to pay for.
Alternately, you can find overstock items, clearance sales and even outlet sales. I've even sold some Coach bags my wife bought at outlets for real cheap because of some deep discounts, for 2-3 times the price.
Good luck!