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Serious Sam 3 BFE is 6 fucking dollars. You're asking for 5 TF2 keys for it? Funny guy.
They're currently £1.31 - http://steamcommunity.com/market/listings/440/Mann%20Co.%20Supply%20Crate%20Key - which works out to $2.13, though I think they're closer to $2.50 just now.
Not only are you asking ridiculous prices, you're flat out denying the clear truth.