The ship buy and sell prices are ever fluctuating. Wait until prices decline and then buy as many ships as you can afford. Now wait until prices again rise, whereupon you should sell. Taking advantage of repeated price swings will result in ever accumulating profits allowing you to buy ever larger ships. Not only will you eventually be able to buy the biggest ships, but your status will have increased with each new ship purchased, and you can even obtain a loan to buy still more ships.