Some asked for the use of the Action Tile number "6". There is no mention on the rule sheet.
You can use the Action Tile number 6 instead of another Tile OR as another Tile in the game. In this way you can have a longer game if you whish. Be careful that in a 2 or 3 player game you do not have enough Trade Stations, so you need another rule:
If you have no more Trade Station available you just cannot sell anymore.
In this case you just need to spend very well your "move without selling" actions just to avoid to find yourself at the end of the game to have nothing to sell because of the run out of your Trade Station. In a 4-5 player game there is no need to apply this rule.