I generally use Depot I for important stuff, Depot II for items I am selling (market) and Depot III for NPC general loot.
I put some backpacks in Depot I to keep it organized and to help me remember where things are. For example, I use the Jewelled Backpack to put rares, in the Crown Backpack I put equipment, in the Expedition Backpack I put quest related itens and so on.
After a hunt I simply throw items I want to sell in the market in Depot II and general loot (rashid, djinn, other) in Depot III. Depot III has backpacks inside it, so I remember which NPC to sell, for example (camouflage backpack - rashid, green backpack - djinn, ...). For example, I put one camouflage backpack with lots of camouflage backpacks inside it.
Then, when I want to sell the items in the market I simply open depot II to see what is there and then use the market for the most part. To sell NPC items I simply grab as many backpacks I can hold for that NPC and go sell.