NP, as I said, it depends. There are some trades where it is possible to be ehrlich, but some – I am not sure. So, you need to learn the economics of the trade before engaging in it.
Let’s say you are selling something that can be smuggled without paying tariffs, or most use illegal workers who use fake SSNs, or parties pay cash to each other to avoid tax payments – can you be honest despite this? I think, it depends:
1) if the occupation is mostly “business” – that is main costs are due to materials and labor that you buy/sell, then it is very hard. If business profit is, say, 10%, and level of cheating is 15%, then an honest businessman will be losing 5% on every transaction. The only possibility would be to have an infrastructure of honest businesses that buy from each other. This is possible but I don’t think happens often (unfortunately).
2) if the occupation involves payment for your work skills, then it is not that hard. If a dishonest plumber gets paid $100K, then an honest one will be OK with $50K. Even more so for accountants, doctors and even professors. They are all paid good salaries. Yes, they also have yetzer hara and have opportunities to cheat. But it is not a crushing pressure that affects business people where it is hard to earn an honest living.
This is not a new distinction. Gemora in Kiddushin 30 that you mention talks about professions v business. Teaching business does not prevent the kid from becoming a bandit because of uncertainty. I think we agree here, I am just pointing that some trades are more of a “business” and to be careful.