Not just the selling side, the marketing side, the promotional and the production sides - but also, and for me, importantly, how you go about managing yourself side.
Some people seem to think that there is an inherent flakey card which they can play - we are creatives, we are not bound by the same rules as others! But, if what you are doing involves you employing professional services, and paying for those services, you are bound by rules.
I learned recently about someone who I respect as a service provider being put into a very big financial hole by people not paying their bills on time.
You don't get a pass on this, everyone - you employ a service, you pay.
If you think you can move the money around, and get someone to carry you for a little while, then you have no business doing this.
Here's how it works for indie creators - you will pay quite a lot of money at some point, for some aspect of your comic book production. Then, hopefully, you have the means to use that property or creation for ever more to make that money back. It may take years, it may take forever, but that's the idea.
You do not hire someone, use someone, employ someone, and then not have the money to pay them, take longer than the agreed upon time to pay them, or ignore their requests for payment.
Also, while I'm on this, if someone hires you to produce work, and you take the money, but don't produce the work, you have no place claiming to work in the business of comics. You are a shyster, using good will to make a buck.
I have a group of people who I have worked with on my projects, and they have always been incredibly professional and all round great at what they do, and how they go about doing it. I may not be Marvel or DC, or Image or Dark Horse, but I do know two things - one, I will use people who behave like that as much as I can, and two, if I move up, they are coming with me.
That is how the business side of all creative work operates.