Finding something with a high profit margin to sell on ebay is getting harder and harder. The way most big sellers do it is to use a low margin, high turnover business model.
It involves spending a lot of money up front on volume buying to ensure you are getting the best possible deal on your stock. Then you have to keep your margins low to make sure you are supper competitive with your competition and rely on selling at a high turnover to produce profit.
With large wholesalers using ebay as a marketing platform, it's a damn expensive way to enter the market, but if you are going up against competitors who do this, then it's the only way you have a chance of competing.
The other avenue to chase is the niche market, selling products that are not saturating ebay, and everyone cutting each others throats to make a sale.
Niche markets are not easy to find always, but if they were they wouldn't be niche markets for long!