While leveling, I found myself most likely to buy a few items in particular: weapons, jewelry, and underwater weapons. Let's take a look at four jewelry items that levelers frequently buy
Garnet Copper Amulet of Might
Garnet Copper Ring of Might
Garnet Copper Stud of Might
Why do players buy these items? What do they have in common?
For starters, they are pieces of jewelry that are better than fine (though the fine quality ones sell just as well as the masterwork ones), so players are buying these to upgrade their questing jewelry as they break into the 15-20 leveling bracket.
What else do these items have in common? Oh yeah, duh! MIGHT. Why do new players light having more might? Because it's the one stat they understand to equal more damage on their character, and they've played long enough at this point that they know what a big increase even +10 might can be.
The final thing these things have in common is that they are all of similar levels. So players are likely to buy one and then search for the others in order to complete their gear set.
These three qualities which all four pieces of jewelry share touch on the three wants that all players have:
Upgrades, more damage, and OCD.
You see, players always want to have better quality loot, an increase to their damage, and they like all their gear to be equally kickass.
That's why these mighty items sell remarkably well. As players reach higher levels, they start to see the effectiveness of other stats, such as condition damage and vitality. It's up to you to do the research, but start with what you wanted as you leveled in order to get a feel for the marketplace.