There is something I do not get: if I choose the currency "WMZ / Paypal" instead of keys, it is $16 (everytime, it is $2 more expensive than DH Store), so he sells cheaper for Paypal than for keys.
No, Im on my phone so I'll keep this brief. Dunders way of getting games this cheap is exploitive of the black market of keys.
First, Russian Sellers sell their games for keys because they resell those keys for profit on market. i.e A game that he could sell directly for 16$, would cost about 17.25$ in keys because of the valve cut on the market.
Secondly, keys on the black market are a whopping (including valve cut into calculations) .30c cheaper. So if you buy a game worth say 5 keys (that you got from the black market), the makes about 10$ from the sale of those keys from the market and you got those keys for 8.50$.
This essentially results in the possibility of buying games even cheaper then they are on the Russian steam store.