If (s)he sells to thousands via WhatsApp and word-of-mouth good old police work will have him/her bagged shortly.
WhatsApps only claim to be private is end-to-end encryption of messages. The moment a polkce officer opens the phone of a buyer they can identify the seller. Same if an undercover cop hears the word-of-mouth and buys.
This is just ordinary good police work that I whole-heartedly support.
If one child abuser can sell his contents to thousands via nothing but WhatsApp, word of mouth, and a Bitcoin wallet, what will you do to fight that?
I’m all for freedom of speech and freedom from fishing expeditions and what not, but if you don’t deter these things they will grow.