Yes, I've developed and sold quite a bit of software myself over the decades, from shrink-wrapped retail software (back when it was sold in stores) to (mostly) libraries intended for developer use.
Most of the advice I have from my experience boils down to "learn how to run a business". There are tons of resources that cover the basics and, as a one-man operation, the basics is all you really need.
Aside from that, things I have learned:
0) Leverage the strength you have as a one-man shop. That strength? Flexibility and the ability to have a genuine relationship with your customers. The big guys can't do either of those things very well (by their very nature). If something isn't working, you can pivot on a dime -- they can't. That's a superpower; use it.
1) Keep it simple. Make your software excellent, but not complex. Use simple bookkeeping techniques. Use a single, simple, monetization method, etc.
2) Have an accountant, and take their advice seriously. Just because you're a one-man shop doesn't mean you can skip this.
3) Don't try to be all things to all people. Identify the specific need you're trying to address and laser-focus on that.
4) Try to make everything you do pay you in more than one way. This can be obvious, like reusing code in multiple projects -- or less obvious, like selling an article discussing some unique aspect of what you're doing.
5) Don't have a single product. If you're nearing the ship date of one and don't already have another well along in the pipeline, you're in danger. As best you can, try to have products that complement each other such that when it's hard times for one of them, it's good times for another of them. (For instance, if you're into selling clothing, have both warm and cold weather offerings).
6) Don't fall in love with your product. If your customers are telling you that the part you love the most sucks, then change it no matter how great it may be. If the product isn't selling well enough to justify the expense of continuing to sell it, kill it quickly.
7) Develop a real relationship with your customers. An honest one, not a fakey business one. Talk with them. Listen to them. Take them seriously. Be a human being, not a company. Be honest with people, both in your business dealings and in how you represent your products.
8) Don't go toe-to-toe with the Big Guys. They'll squash you like a bug. You want to address markets that aren't big enough to be interesting to them. A minuscule market from the point of view of Microsoft can be amazingly lucrative from the point of view of a one-man operation.
I know that I've missed a ton of things here, but these are the main points that have worked for me.
