To add to what @Acidexx said, I would recommend buying seasonal items that usually increase in value a month or more after leaving stores.
For example, in the past week or two, I've made around two solids' worth in diamonds (2,000) just from reselling Yeti Claws, Streaked Wing Hair (both of which were sold for gems during Night of the Phantoms), and Jamaaliday Tail Ribbons (which were sold for gems during the Jamaalidays).
I sold these items for 15 diamonds or lower so that people would be more likely to buy from my shop since this price is technically below the listed price of a den beta. I also plan to do this with Clover Leg Warmers, as these tend to increase in value after leaving stores as well.
This (and fashion shows, lol) has been my main way of gaining worth on AJ, and it works quite well—especially if you advertise what items you are selling here on the Wiki.
Sorry if this makes zero sense; I'm a little tired while writing this. Hope this information helps!