I noticed a lot of people sell prepaid virtual credit cards for Bitcoin in here. Anyone knows how the sellers go about making the cards?
There are websites or apps providing VCCs for free like Oxygen wallet, Lime, Udio... Or some persons basically receive VCCs as rewards from their online activity, etc. the same way people receive and resell Amazon vouchers, Bing rewards...
Doesn't Visa/Mastercard have KYC rules?
Up to $150, not really. That's why there are still places to buy prepaid cards without KYC (i.e. PCS)
And isn't it very expensive to become a credit card issuer?
Legally as a company? It's a nightmare between banks, Visa/MC, regulations, attorneys, partnerships, and so on...