There are plenty of individuals selling them on various trading sites, and there's usually a reasonable amount of competition on prices. Marketplace is pretty standard, though.

there's a lot of key sellers through paypal, though how that would work with currency conversion idk

i've always bought through

http://steamcommunity.com/id/toreutic/ - $1.85, he pays fees and you pay goods and services
http://steamcommunity.com/id/SapienS_SFC/ - $1.79 you pay fees, gifts
https://steamcommunity.com/id/same2me - idk but sold me keys

opskins and the backpack.tf forums are decent options, and trade tf's community list shows the site tf2buy.com is an option as well.

There are trading websites like backpack.tf that have people selling keys for real money.

http://forums.backpack.tf/index.php?/forum/27-team-fortress-2-cash-trading/

Basically you will go first, but you'll save at least 30 cents per key key which adds up, what you want to look for is backpack.tf trust. For example here's mine: http://backpack.tf/trust/76561198121890168 An example of a lot of trust would be somewhere from 30-200 posts.

You can also copy paste people's steam links into steamrep's website to see their reputation there.

You can add me too

Thanks for the posts guys

Edit: [redacted]

i'll have 14 available in my inventory in 7 hours (pending trade from march 13th)

Backpack tf and sourceop trading forums are the best places imo to buy or sell items (including currency like keys and metal) for real money.

I'm actually selling 25 keys on backpacktf atm so if you're interested hit me up on steam

opskins has tf2 stuff now aswell

yes but opskins is expensive

Ive always got keys coming and going, just buy off someone from tf2oupost. That will be your cheapest option. Just make sure they have alot of rep and look for a good price. 1.85 is about normal.