The availability of the specified MTI is also dependent on the trading post level. The way I believe it works is that the full amount of the MTI produced is divided among the ports slated for its availability. Ports with a higher trading post level get proportionately more. Also other companies will purchase MTI from open (unconquered) ports so as to deprive other companies from a source.
If you do not have enough money the auto trade ships will come into port and leave empty. As money becomes available when reaching other ports they will purchase whatever they can to make a profit. Worst case they will come back empty and you will have paid the commanders salary (which simulates upkeep, rations and crew costs) for the entire trip.
It is very important that you not over-spend in the early phases. You end up getting off to a slowwwweeeerrrrr start.
Greg.