The problem is that many domainers have no idea what businesses are looking for in a domain because they do not own their own business or they shy away from making real investments (e.g. they do not participate in EAP to get the real good names). They do not treat it as a business but as a hobby. That's why they make bad buying decisions. However even advanced domainers make bad buying decisions from time to time and they let them expire.. thats just part of the game. You really need to be highly critical and conservative, especially when you are investing into something so speculative.
One other thing is that every extension will have certain types of names that sell better. One only has to look at what is hot right now and has sold in the past as well and transfer that knowledge to new extensions to find real gems. Its doable even if you lack the knowledge of domainers with 10+ years experience. Just do some proper research and think real hard about the commercial purpose. Most hobbyists dont think about the commercial purpose, but that is so critical.