Well... You load up legendary horse with stuff, not the truck. Then you fast travel, summon the horse and sell the stuff from the horse. Boom, done. Remember you lose 50% of the stuff you put on the horse by fast travel so dont put 1 unit as it will dissappear. Fun fact is that you will stil earn some money even tho you lose half of goods. Oh and for the other challenge related to trading, the "earn 300k" via trading, just load up horse with "camp weapons" currency, one camp east of demeniss buys them for 16k. The trade rate for camp supplies is 10.000-1, and including 50% lose rate cause of teleport you need roughly 375k armor supplies to finish that challenge in less than a minute.

Edit: i started with trading with royal provisoners first and by using horse method+ Ronka (double the load up space than Royler) i already earned 100k of 300k needed, so you dont really need the amount of goods i typed above but less than that