I think in order to trade, you must have a premium account.
A suggestion on getting one, wait for a TF2 item sale. You can upgrade your account to premium by buying any in-game item, so I personally waited till a $0.50 item went on sale for like $0.13 and got a my account premium for cheap!
Alright I shall try that hopefully it works as it has been possibg me off -_-. I should jump on the SpA server more often xD
*Edit* I have found out that you do have to have a premium account so that you can fully trade items and not just have a semi trade system, thanks for the info.