You need to look at latest trends. What's hot at the moment and potentially offers almost a 'quick' buck over a short term, before the next bandwagon hits town!
Current 'hot' areas are iPODs. I have looked for reputable wholesalers of the units' themselved, but given up as there are too many dodgy dealers out there. But think of another side to this coin. The market for iPOD accessories is massive due to the volume sales of iPODs themselves, so maybe you can cash in on this too?
Site statistics can be a valuable marketing and research resource. Look at the Top 10 viewed items on ebay and you will see that the moment there are some unlikely items. iPOD is not actually mentioned at all at the moment, so maybe look at the other items and build on this knowledge.
I use the site statistics on TradeMe in New Zealand to see who is viewing what (baby items suprisingly leads here), what the peak time for site viewing is and optimal auction closing time, to ensure I get the most 'hits' possible.
Think about what people are using; what people want; how much are people willing to pay for your goods; and what your competition is. When you can crack this you should have a profitable business!