The system itself will not allow this to be done. There are admins in a large online store. They act as guarantors of the transaction. They often make control purchases themselves. Negligent sellers (who either send low-quality goods, or do not send anything at all after receiving money) are immediately kicked out of the store. I even know of cases when fraudsters were physically punished.
Online stores selling illegal goods are, in fact, virtual supermarkets. Ads for the sale of drugs or weapons are placed for money. A place on the site costs from 300 to 1000 dollars per month.
By the way, you can also find a high-paying job here.
For example, a drug courier. Salaries are really good. Those who deliver large quantities of expensive drugs are sometimes paid up to 10 thousand dollars a month. Couriers even have a paid two-week vacation once every two months. But it is quite difficult to get into such a job. We need recommendations.
In addition, you can earn a fee for publishing articles on the Darknet. For example, to offer some website an article on the topic “Car theft and legalization”, “Hacking a personal page in social networks” and so on. The fee is paid in cryptocurrency.
By the way, all purchases on the darknet are also paid for with cryptocurrency.