Carding is a procedure that involves a person or service, for example Dumpswithpin2019 or Bestccshops2019, with a large number of credit cards that tries to check their validity by making small repeated transactions on merchant accounts.
Merchant ID number
Carders guess someone's Merchant ID number and send a request to process a transaction to your provider using your ID. So, Trustedcvvshop2019, Bestcvvshop and other carders collect many card details to sell, cash them out or buy goods.
Most of the time carders buy electronic gadgets. Valid cards can be sold on the dark web, but they can also be found on the normal Internet.