XCOM: Enemy Unknown Cheats
Not easily since you need changing values to find and edit things.
For those that don't understand cheat engine, install it and go 'All Programs/Cheat Engine x.x' and click the 'Cheat Engine tutorial'. It teaches you the basics of the program and how to fiddle around with stuff in memory. If you make it to pointers, you can pretty much edit any single player game out there.
Not what I hoped to hear *sighs* I guess I'll start over again, and remove the negative values! Thanks anyway!
It has instructions in increasing difficulty that can really help with understanding, and associated forum-posts should you need them. My first few tries I made it through all the basics (which is all you really need for this game), and got stuck at the multi-level pointers.
In this game, many values change between missions, unless you find the pointers.
For the more daring among you, once you find a single value using the Gray Market, then right click on it and click 'Browse Memory Region'
You'll get a hex-representation of the memory, with your current address in the left top. All the items you have are in that region, separated by zeroes. Since everything is 4-bytes, a value of 100 would be '64 00 00 00'
If you feel comfortable, you can use for example calculator to convert the in-game values to hex and look them up in that region. That's how I do it. Change it to 'FF FF 00 00' for example for 66535 items. That should be more than enough.
Be careful when doing this, changing the wrong address could result in a crash, so save often. If you do it correctly, you'll see the value change as soon as your mouse goes near it in-game.
Also, edit only the values of items you already have. Some values may edit live specimen and possibly break your game. If nothing changes in-game, just change the value back and hope you didn't break anything.
This is the easiest way to run through all items in your storage without having to search for each separately.