First you should save about more than 5,000 coins or more. This helps with buying items from the market, if you have leftovers of coins use them for Maintenance, Upgrading cars, And the gumball machine. Remember to Sell your entire cargo once it's full.

Now when you are done preparing, go to the next station and buy everything on the market if you can't buy the entire market, go to the next station and sell everything you have in your cargo. Now buy everything from them. You can also make a quick buck if you have something that is on demand and then selling it ton the station that want's it.