Pssst! Got an iPhone for sale on ebay? Or perhaps you have another item that you are wondering, have I priced this right? By browsing over 30 million ebay auctions, Bidvoy can pinpoint the answer precisely. The Bidvoy API is in beta, meaning among other things that you need to email them for a key, according to the API documentation. They've also created a repository on Github.
You can see a huge amount of information in the above screenshot--this one focused on the iPhone 4s as an example. Here's an interesting wrinkle: you've no doubt pondered the price you might sell something at. What about the time you sell it? Bidvoy says it has used mathematical formulas to figure out when the cheapest and most expensive times are. If you're the seller, presumably you want the expensive time, while buyers want the reverse.
Bidvoy is the kind of app that makes you wonder, how did I manage without it? In addition stats you would expect, like the average price and how many auctions there are for your item, they also give a weekly trend for the price. (Yikes! The price is falling; better get that phone up for sale fast.)
But most interesting is the price margin stat (see the $18.10 figure midway down the right hand side). Presumably that iPhone that you hypothetically have for sale is somewhere between mint condition and pretty banged up. If the average price represents mostly pristine phones and yours has a few dents, what do you do? You can't even really tell what the average condition of those phones are. Here's where the price margin helps: it gives you the difference between the lowest and highest prices. This suggests your phone's price depending on condition should vary by no more than about $9 above or below that average price.