This is the kind of information I wish I had before buying my first domains. I probably could have avoided wasting money on names that had little chance of ever selling.
I also recommend keeping detailed notes about every purchase. Write down why you bought the domain, who the target buyer is, and your expected resale value. Looking back later helps you identify patterns in both good and bad decisions.
Many beginners underestimate how important market research really is. Spending an hour researching can easily save hundreds of dollars in unnecessary registrations.