Or we could use the Steam's community currency.
A TF2 key is (constantly) about 1.75€ (a bit more than 2$).
In the holidays it's 1.50€ tho (I think it's 2$), so, the faster you buy it, the less you pay.
It's widely used to buy games from traders at very low prices (Bought HL2 and EP1 for 1 key each = 1.75€ each, outside sale times).
I'm quite trusted (in my little) so I can hold the money in the meantime.
Everybody could give a key, and the winner takes them all.
You don't need to have TF2 premium, just MAKE SURE TO BUY THEM FROM THE COMMUNITY MARKET (If you buy it from the official store it won't be tradable for a week for security reasons).
This obviously also allows for a different currency to be used, and will solve that problem.
But you need to have a Steam account and to put some money on it (Minimum 5€).
You can put money via PayPal or credit cards, or you can get a Steam Wallet Card from Gamestop.
I can also help the winner get some games with those keys.
Sadly for you, you will have hard times selling keys for real life money, unless somebody buys it and pays you via Paypal (not THAT safe tho).
But buying Steam games is easy and safe.
To be fair for the competition, I won't be partecipating
If you like the idea, http://steamcommunity.com/id/essgeeeich/
How can you tell I won't run away with the money?
1. It isn't real life money, but a virtual currency. You can't buy a sandvich.
2. Trading Ban is possible (And has been done many times for scammers).
3. I put my face on it... Er, accounts.