Speaking as someone with a graduate-level education in mathematics, I definitely can't make change as quickly as an experienced cashier, and I'm sure a drug dealer would be able to judge the profit or loss expected from a series of transactions more quickly than I would. Some respect for those skills is warranted, I think, even if they just boil down to being fast at arithmetic.

parent is right, its not really impressive, but these guys didn't know the maths they were doing, they didn't even learn or try, it was just natural to devise rules, ratios, conversion because they just wanted it was inspired by a strong desire. I'm sure some dealers have nice economic skills.

The fallacy is in equating criminal activity with stupidity. This fallacy might stem from assuming that the risk was immeasurable and the profit therefore potentially negligible (bayesian inference).

Arithmetically inclined individuals being advantaged as sellers leads to survivorship bias. It's not actually surprising that a salesman can do that. It's surprising that drug sales can score huge gains and that some people see hardly any alternative to make a living even if small time dealers hardly make a profit because of oversupply in competition, much less if they are addicted.