Think about how you will sell a domain before acquiring it. Read which type of domains were recently sold. Namebio is the most popular place for this, also you can follow
https://www.namepros.com/threads/report-completed-domain-name-sales-here.83628/ However as always, everything has 2 sides like coins, visible side and invisible side. Be skeptic especially on public data. I mean unsold domains are never reported. All we can find reported sales and those are just a piece of total sales, not the whole.
You will create your own knowledge by trial and errors. Try all types of domains and then you will know what to focus on. Anything can work well if you do it right. Success usually is not hidden in what to do, is rather in how you do. Focus on only the things you do right or better. If you are good at hand registering focus on it (your verbal skills maybe outstanding), whatever you do better focus on it. Know your own talents first and then search for the areas that your talents can bring you the most competitive advantage against to the other players of the same game. Every person has unique talents and background. You may be good at math, if so, register/buy more math related domains. You may focus on geo domains about the locations you actually know or lived in, or industries you worked in before, or areas you studied, or about your family business if your parents are/were business owners. Technically you can register/buy in any type of domains. But if you choose the domains related to the areas you know, you can have better ideas on what to do with those domains, how to sell them and for which price. Never buy or register randomly just to raise the number of domains you own or just to spend your budget faster. Buy slowly to sell quickly. If you sell 10% of your domains each year, you will need only 10 domains to sell 1 domain in 1 year. Think before buying/registering not after. Remember anything can work well if you do it right. Stop looking for the best thing to do, look for the best ways to do something that is done by others. Focus on the things that you say/believe you will do better than others. I hope I could help.