I have never understood how it is more profitable to drive a box truck 1,000 miles and sell things in Mexico as opposed to selling locally, but somehow he does well at it. Maybe he's bringing something back with him, if you know what I mean. Just kidding, he seems like he's a decent guy.
I can think of 2 reasons:
1. More shoppers; Were I live the Mexicans will by everything, the "Americans" only pick.
2. Less competition; How many Americans are going to be brave enough to sell in Mexico?
I would think one would need to drive the largest truck one can get your hands on though to make it profitable at 1,000 mile round trip. A 26' U-Haul is about 24' x 8' x 7' volume, so if you were just taking clothes, appliances, household goods, the smalls; you could probably pack the contents of 3-6 units in there.
At 10 MPG, thats 100 gallons of fuel, so worse case $4 a gallon $400. The contents from 1 or 2 units could cover that (assuming you sell-out) then you have 2-4 units worth of inventory to cover the trip (I didn't include rental fees, for all I know he owns a big truck)
Hopefully he can pay for all the contents by selling 1-2 units worth of stuff & have 1-2 units worth of stuff for profit (big IF, since no idea what he is paying for inventory, or if paying employees to help)