what are tf2 keys?

Yar never ganno get it outta me

Because you can get easily "fooled" if you're not into it, it's up to you. That's just my opinion.

Because there can be quite a significant difference in TF2 key prices and actual value of Steam games. Some people may try to sell you a game for a certain amount of TF2 keys when you can actually purchase it cheaper if you take a good look around.

EDIT: I've been in TF2 keys for game trading for quite a while, so if you need a bit of advice, add me. I can help figuring out if a deal is fair or not.

Should have added that in, thanks :P

Yep, once you buy them they get added to your inventory under "Team Fortress 2" as tradable items.

Be careful though. 15 TF2 keys for a Bioshock Infinite is suspiciously low. If the person offering it says it's a Steam redeemable code, then take extreme caution. If they request you to go first before they send you the code, it is likely a scam.

Whenever you want to deal with tradable items for codes, always ask for a trustable middleman to help you out with the trade.