Anyone having experience with Steam key reselling?

Was thinking about people selling Steam and other game key on G2A and similar sites... some of them have thousands of transactions, so they are making a sustainable business out of it. How do they manage to do it? Simple arbitrage, scouring the web for discounts? Or are they just scammers using stolen credit cards? Something eludes me in this market, but I think it could be very interesting.

Stolen CCs

CRT said:

Stolen CCs

Click to expand...

Just that? I don't think so. I've bought close to one hundred game keys and just two of them have been revoked. I think it would be a higher figure if they were all stolen.