Buying steam games at lower price ?

I have a steam friend and he is "buying" me games for lower price.
He gifted me a 60 $ game and asked for 10$.
I have met him in person and gave him the money. I bought 5 games so far and they all work.

i know that he is not just generous , he is somehow exploiting , buy HOW?
Update: Reply @Mick O'Hea

Yes , i think his brother is somehow getting keys

If you need to know he sold me black ops for 1 000 (Serbian) dinars/RSD or $13.0787
monday night combat & bbc2 for 19.6181
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