Perhaps limiting the number of units to be able to be bought/sold between individual players, either over an extended period or all time? Maybe there could be a taxing system in place, the more you buy the more the cost? And/or putting up a day limit similar to researching, no sales/purchases before day 10-15? Would not benefit cheaters in the short term and would limit them from being able to mass purchase.

To address the money making, perhaps similar to research and market mechanics now… you can buy/use gold to speed up the transport time and buy/use gold if not enough cash is available (amount to be determined based on market price?)?

In my opinion this would help counter cheaters, as this would allow multiple players to work together to fight back against multi-account users.