Just wonder how does works backpack system
After make few trade i got around 360/303 items in my bp and some alert:
http://i.imgur.com/HR4gF8J.jpg

But almost 200 of them was steam gifts and on front page of shop says i have more (3000 like all steam user) space for gifts, or no?
http://i.imgur.com/NDfJbdc.jpg

How it works?

Unfortunately, the system can't tell the difference between inventory allocations, so, it calculates everything as one inventory, that inventory size is the size of your TF2 backpack. In your case, this is a cap of 300:-

http://backpack.tf/profiles/76561198077130520 - "9/300 USED"

If you wish to increase your global backpack size, add some backpack extenders to your inventory and then post a request for them to be applied here:-

http://dispenser.tf/forum/view/5724/

Pain? Yes.
Makes it relatively easier to develop and maintain? Yes.