I have acquired a lot of nice blue and green equipments ~lvl 20. I have no use for these since my class can't equip them. Should I just npc these or salvage them? What is the most profitable way?
The most profitable way to get rid of rare equipment is often the Black Lion Trading Post. For common equipment, the trade price is not even high enough to account for the cost to post the item. In those cases, it is more profitable to just sell them to a merchant.
I would just sell them to a merchant unless you really need the materials that they are composed of, the essences of luck, or are working on the monthly salvages achievement. Salvaging armor pieces usually nets less value than simply selling them.
Most common equipment is better off salvaged than sold. Even if you're not going to do crafting, you can usually sell the components you salvage in the TP for more than what you can sell the original item for. For 'green' items and above, better to sell them to the merchant or TP. You can check the prices in the TP wherever you are (no need to be at the trader for that), so you can see what's better for you.
It really depends also on what you are aiming at, for example Legendary weapons require different behavior, but basically that's what I do:
76+ rare/exotic weapons: save for throwing in Mystic Forge for legendary precursor
68+ rare/exotic weapons/armors: salvage with Black Lion Kit for Globes of Ectoplasm (to craft new 80 exotics and legendary)
Then everything selling for good at Trading Post is sold here (I usually sell what is already required by someone)
The white items salvaged (sometimes blue too, if you need the salvage achievement), the rest sold at the merchant
Even if you are not aiming at legendary, salvaging and forging can get you some nice items and components which you can sell at an higher price, compared to the original item
Hope it helps!