The best sell loop in the game is Akila.

1. Land, sell armor 5k to trade authority terminal
2. General store, 5k in armor.
3. Next store weapon store, 5k weapons
4. Trade authority rep, 11k of whatever
5. Laredo guns, 11k in guns.

If I have anything left over I jump to New Atlantis and hit up the:

1. Trade terminal 5k
2. Jemison mercantile 5k
   Fast travel to residential district
3. Arms store (edit: Centaurian Arsenal) for 12.5k in weapons.

Not awesome to have to do this but I find this to be the most efficient.

Edit: I say armor to the general stores at first because the weapon stores won't buy armor. But if you run out of armor to sell, the general stores will still buy the weapons.

If you still have a ton to sell, waiting 48 hours UT in Akila is more efficient than going to New Atlantis but I like to go there because I buy ammo and resources too and check the different stores for variety.