So, you want to buy/sell a second hand iDevice (or other Apple hardware).
In my case I wanted sell an old Airport Extreme Base Station which has a model number of A1143. The problem for me is that with that model 2 versions were made, one with Gigabit Ethernet & one without. (Potentially the difference between a sale or not).
What I realized was that it is VERY difficult to find the actual tech specs of your Apple devices unless you have the original box etc.
After a bit of a search I found Apple's tech spec site:
Just put in the serial number & get all the details of your iDevice - brilliant.