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When it comes to armor, I find that masterwork items get me more profit than rares of similar levels. Sure, the rares can usually net me a larger amount of silver per transaction, but the masterwork items move so much faster. Especially at key levels when players look at their character and go... "crap I need to upgrade!" When they get to that point, they try rares and think, "Damn, I can't afford six of these rare pieces of armor and a great weapon. Let me buy the great rare weapon and then fill out my armor / trinket slots with masterwork pieces."
When it comes to weapons, I find that rares do far better than masterwork. In most cases, there are very few masterwork weapons even worth flipping because they are all selling for practically the same cost as the vendor amount. With fees you usually can't even get close to a profit.
That's my reasoning for why those two conditions exist. Think I'm right or wrong?
In the gold guide / forum I outlined a very simple strategy for turning 1 gold into 50 over the course of a week using these ideas plus a few others. Check it out!