Then look ahead and try to draw conclusions based on current levels of pricing, planned development and sales activity. This would require a fairly long-term view. And you should remember that the prices of some collectibles may seem to bear no relation to rationality. Values may shoot up inexplicably before crashing down again. Watch these downturns.
Fashions have a way of repeating themselves.
And there is no better price for a buyer than the lowest one. Of all the collectibles, stamps and coins may be the most appealing. They are easily moved and stored, and they have a long history that can be studied. But whatever you might pick, it should be something in which you have an interest. A hobby, even when pursued with a profit motive, should engage your imagination.