Popularity: Buy well known collectibles
If you're collecting coins that no one else is collecting, they'll never have any value to anyone but you. The example I like is the vintage movie posters. Let's say you have a mint condition Gone with the Wind vintage movie poster and you also have a mint condition vintage movie poster of How to Drive a Military Tank, which do you think would be the easiest to liquidate? It goes without saying that the Gone with the Wind poster will be easier to sell because it is more popular. This is why you want to buy well known collectibles.
Quality: Always buy the highest quality coin you can afford
Coins are graded on a scale of 1-70. 1 is like the penny we put on the railroad tracks to get run over by a train and 70 is the finest known. With today's technology most modern coins are grading out 69 or 70.
Changeovers: Keep an eye out for special opportunities
A changeover is when something different happens to a coin. Example: The last year of a design, the 1st year of a new design, or for a special anniversary issue. When something changes with a coin it can become a huge opportunity. Collectors love firsts and lasts!