First you cannot sell anything unless you figure out your target market. In order to do that you need to find a problem in the target market that isn't solved or isn't solved very well, then solve it and make a product and/or service for it. Make sure it is a large market or else you will niche market yourself into a hard to grow area.

Second if there is a goldrush for something, instead of becoming a miner, become a general store and sell them tools, picks, and shovels instead. If you write programs write libraries, development tools, templates, websites and ebooks on the subject, and other things. Remember that in most goldrushes miners will find fool's gold or not find any gold at all. But the ones selling them the tools will always earn money.

Third take care of your customers so they become repeat customers. You might have to offer them discounts or free stuff, as long as it helps them decide to keep coming back.

Fourth do a SWATT analysis. Strengths, Weaknesses, Analysis, Threats, Trends. Focus on your strengths, cover your weaknesses, analyze everything you can, look at threats like competitor's products and tech that can make your product obsolete, and then watch trends so you know how to avoid threats by innovating your products and services.