What I've learned:
1. Not all rules matter. A large part of my business is stretching certain rules, either from the marketplace, or from the source (e.g. a store that doesn't allow resale). That said, you can't get away with breaking rules unless you have a very good understanding of why the rule exists, who's motivated to uphold it, and generally what the risks are. Don't screw over customers.
2. There's a lot more to be made by taking risks than there is to be lost. I've easily lost over $1k multiple times in various ways, but when I "win" it's to the tune of 10 or 30 times that. Take smart risks, only where the realistic upside justifies it.
3. Be willing to pay for information. There are courses out there in almost any topic. Personally I've largely carved my own path and paid very little , but I'd still recommend courses for others. Also read a lot of whatever free information is out there, and network with people who have more experience.
4. Don't do too many things at once. It will kill you. I've been full time in college and it's extremely tough to balance everything. Delegate as soon as you can afford to, anything others can do that doesn't take a lot of brains pay people to do.
5. Don't be afraid to scale, but do it slowly. My first purchase of over 10k was 6 months after I started, iirc.
(Several of these are probably specific to this kind of business, may not be generally applicable. Startups have a much different road where profitability isn't the most important at first.)