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If you don't need Serious Sam 2, I'd rather accept Serious Sam 2 as 1 key and do it for 10 key.
(SS2's lowest steam record is 1 usd but I can go for it for 2.5 usd)
Serious Sam 2 https://www.goclecd.fr/acheter-serious-sam-2-cle-cd-comparateur-prix/
it was once on steam at a very low price so it is worth at least 2 keys sorry.
Eh honestly there is an upcoming Spring Sale and right after a month there is a Summer Sale. I'd bet it'll be in the bottom as low as 1 USD once again. Still, np for me.
Can honor your offer.
5 TF2 key
Final fantasy XIII / Final fantasy 13