This tier of trader would only be accessible by selling and buying from traders, similar to how leveling up vulture works.
Their wares would open up in this level to more end game equipment which is currently impossible to buy.
In order to make it a grind, my suggestions on their rep points required would go as:
Handshake - 14,000 rep
Gunny - 9,000 rep
Lab Rat - 6,000 rep
Artisan - 5,500 rep
Turncoat - 7,000 rep
Banshee - 6,000 rep
Each trader would sell their respective trades of either gear, guns, or ammo.
Handshake - selling 3++ rigs, belt bag, MM Trunk
Gunny - Black tip ammo, MOE buttpad, high range mortar, Vudu
Lab Rat - Fenethyl, Strychnine, higher tier food and water items
Artisan - BP 7.62x39, SR and AP 7.62x54R, Modded SVD's
Turncoat - M2 Rig, BS and BP 5.45x39, Rush bag
Banshee - MM Ammo Box, M61 7.62x51, .300 BLK AP
Restrictions:
The rigs and belts can only be bought once per reset
Ammo can only be bought in stacks of 30 per reset
Rush bags can be bought 3 times per reset
Weapon parts can only be bought 3 times per reset
Any ammo will have a steep price of over 25 dollars a round.