You don't need to play the game, but you need to know about the items. If you're willing to put in the effort, it is worth it. I turned my $50 inventory to $3,000 in two years, probably would be worth more if I didn't cash out a couple times.
Valve made inventories private to friends only by default. Also when you but something from the Community Market or Mann. Co Store, it is not tradable for 7 days. It is harder to do, and harder to find profitable trades, but not impossible.
Currently the two best free scanners are Scan.TF and TF2Endgame.com