Here are some resources to at least get you thinking, along with some thoughts of my own:
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Sort of the granddaddy of shopping info. In fact, maybe a little too much info unless you already have a niche in mind ... but if you do have a nich in mind,
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eBay's pulse and Want It Now pages can be gamed by spammers, so bear that in mind ... but they are still worth checking out to see trends. Terapeak is a third-party research service for eBay that can give you volumes, average price ranges and trends for a given product or product niche, and I highly recommend both the free or paid versions, whatever's in your budget.
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43 Things isn't a shopping site, but a goal site. Look what goals are continually at the top, sell items that are conducive or related to achieving those goals and promote your listings via a mini-site about that goal (little bit of work but worth it if you know what you're doing.)
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Look for forums in various niches and see what people are discussing. I like to look for hot products, and then instead of looking for the hottest new product, see what people are putting up as cheap alternatives. I did very well with this strategy in the wake of the release of the iPhone, using the Let's Talk affiliate program -- never bought or handled the first phone myself but made a nice commission promoting 'the next best to the next best thing' phones from there during all the iPhone ruckus.
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Find what's hot on ClickBank, which is a digital-only marketplace, and dig up some physical products that complement those digital ones. Some cheap eBay auctions, a small website with some useful info on it, and suddenly you have two potential revenue streams -- one selling physical products, and one making affiliate commissions from the ClickBank book.
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Get informed on trending niches at these and other 'social shopping' sites.
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Search these sites for some of the most incredibly low prices on new and refurb merchandise out there. Why would you do that? To get an idea of what NOT to sell, because those products will be flooding eBay within 30 days. Most of the items that make it into the 'so cheap I can't believe it!' price range are either about to be phased out, obsoleted or seriously upgraded to a new version that everyone will want. So manufacturers and distributors and major retailers, who are always ahead of you on the information curve, are dumping them in anticipation of the new stock (there are other reasons, don't get me wrong; I'm just saying that this is a common reason that major deals exist online, and one should be wary of it.)
Hope this is of some help. Click here to buy my ebook on niche research ... no, just kidding. ;)
Frank