There's many factors that affect an item's "popularity" on eBay. First and foremost is eBay's search standings, or how eBay determines that your listing should be shown ahead of similar ones from other sellers or not. Clearly for very popular items where there are as many as hundreds of sellers offering thousands of items equal or similar to yours, how high eBay rates your listing determines how quickly it will be seen by sellers looking for that type of item. eBay will not reveal the exact parameters for how they determine search standing for listings and they do tweek that periodically, but there's a few things you can do to get a better exposure. If you are a long time seller, your DSR's (Detailed Seller Ratings) which are computed monthly based on your performance and on buyer's feedback, will be a big factor. Sellers with higher DSR's get better exposure, and sellers who have "Top Rated Plus" standing get the best exposure potential. If you are a new or casual seller, then there's other things to look at. Auction listings get better exposure than Buy It Now fixed price listings, and listings with free shipping also get a big boost in search standing so those two things are important to consider. You may want to raise the price of your item slightly to allow you to offer free shipping. As far as your second item doing worse than the first, there's a simple rule of thumb: if an item you sold did well and you will sell another one, use the RELIST functionality to list the second item instead of using Sell Similar or creating another listing totally from scratch. This will insure that the item title and description and other parameters that may have caused the first one to sell well are totally duplicated for the second listing. One thing that you should do to monitor your competitors is to use the Advanced Search and look for "Completed Listings" of the same product you are selling and look to see how many have recently sold and for what price. Now, as far as the iPhone itself as the product, do keep in mind that if you sold your item during the holiday period, that's when items such as iPhones are in highest demand. Retail stores get 40% of their yearly TOTAL sales in the 45 or so days before the end of the year and online sales probably behave similarly for popular gift items like iPhones. One other point to consider is that the "lifespan" of iPhone and other electronics products is very short and price drops affect sales (AND PRICES) not just of a particular product, but also its previous versions and competitor's similar products significantly. Since the iPhone 5 came out in mid-Sept. it is very possible that many people still bought previous iPhone versions more often a few weeks ago, but in late Dec. the price of the iPhone 5 dropped by over $100 at many retailers, and this may have induced many people to get the newer iPhone 5 instead of an earlier version. Also, Apple recently announced that in the coming months they may release cheaper versions of the iPhone so many people may have postponed purchase plans until they see what happens. Also, it may be just simply that when the iPhone 5 price dropped, it caused the prices of used iPhones in general to drop and if you used a high starting price or have a Reserve price, it may not be as attractive as it was a few weeks ago. Happy selling!
long time eBay seller