What item do you find turns the fastest profit in the Trade Depot

I do just the opposite. I spam purchase complex products. The automated orders almost always pay much more for the items. For instance, bricks will sell for under $300 in the trade but demand over $400 from the NPC. This is effective since the little coin(s) tends to respawn every 5-10 mins. Obviously you need an initial bank roll and a decent sized storage.