IIRC not all items in the game appear on the market, some have to be recovered from aliens.
Regardless, you can alter the .ufo script files - they are in plain English and no programming experience is really needed to change how the game runs with them - by unzipping the "ufos.pk3" file and changing them with a text editor (hint: on Windows, Wordpad works better than Notepad, simply because of spacing/line ending issues).
The .ufo files also control the market among other things, and have plain English comments to help change them.
Also, what exactly are you testing? What are you really trying to accomplish as an end result? If we knew just what you're trying to do - the complete picture - it would make it much easier, and we might also be able to suggest a better way of helping you accomplish it.