@borderlinetrade,
As everyone else has mentioned (by implication), it's not as easy as it looks. If it were simply a case of running some metrics and coming up with figures, then buying some bulk cheap items through Alibaba, everyone would be doing it.
The trend for success on eBay for smaller sellers has not been upward-rising. Even long-established good sellers have been finding things increasingly difficult, to the point that some simply sell elsewhere, give up, or become ballet dancers or janitors or something.
You're going to have to look at more data than sell-through rate. For cheap items such as unbranded iPhone cases, the major players on eBay are Chinese sellers. They not only have ridiculously low postage costs (because China to Australia shipping is subsidised to the enormous benefit of Chinese sellers and to the enormous detriment of Australian sellers), but they can also afford to sell at a lower price still than your prices.
It gets worse. Chinese sellers appear to be largely immune to punitive action by eBay for flouting eBay policy. Partly this is due to the way in which eBay in China is run according to different policies and actions and under a different societal and legal situation. As an Australian seller, you don't have the umbrella of protection that Chinese sellers have on eBay.
So... factor in information such as who is selling particular items, and at what price (including postage cost). Factor in information about what volume of sales are being reached and maintained by your competition, because high-volume sellers can absorb more defects and complaints and refund requests than you can. Not only that, but when a buyer sees that they'd have to return an item to China for a refund, this often stops the buyer in his/her tracks (irrespective of what the MBG states, because more buyers than one'd think actually have no idea about what sort of protection they do have in such cases). This contrasts strongly with buyers wanting a refund from an Australian seller - Australian return address, Australian eBay policy, etc.
Consider all of these things as well as the issue of finding a niche product (or niche specific within a wide range).
