I thought about that back in the 90's - a drug dealer who wanted to accept credit cards for payment could easily set up a porn site with very little content, accept credit cards for subscriptions, and just keep track of which of their actual customers was associated with which subscription account.
Another interesting idea would be to make a mobile game (even a very basic re-skinned asset flip would work), allow for loot boxes, loot crates, or even just an ingame store to straight up "pay $x to get item y re-skinned with z color scheme". Give your customer a digital receipt and have them produce the confirmation number when meeting up to take possession of the illegal goods. This doesn't solve the problem of giving two otherwise disconnected parties a legitimate and believable reason to meet up, but it would make a fairly flexible payment system for dealers. The type of loot box/item bought could correlate to the desired product and the number purchased could correlate to the desired dosages.
There are probably all kinds of other issues with this; I haven't fleshed the idea out much, it's just something that struck me when reading your comment :).
This strategy predates and doesn’t require the internet. I was acquainted with a drug dealer had a cleaning van. All her clients were given cleaning invoices. She lived in a very respectable condo. I’ve heard the IRS can send investigators to figure this out but I doubt they have many resources.
