Once you have had a town building upgraded and traveled down the road in the Road Raid long enough you will eventually run into a town called Stiltown. 

Once you have reached this town you have now unlocked the Trading Post. 

The Trading Post allows you to gain the resource Steel Sheathing as well as some Zombie Coins. To enter your Trading Post you will need to click on the Navigate button. 

There you will see the Map option. Click on the Map and it will bring you a Stiltown on a map with an option to click on your Trading Post. 

Once you enter your Trading Post you can start decorating it in order to increase your Trading Post's Prestige. 

Prestige in Trading Post doesn't effect your customer limit.

Instead, it will effect the number of Steel Sheathing and Zombie coins generated regularly. 

However, you can upgrade the Counter. Which allows you to increase the amount of Steel Sheathing and Zombie coins you can store before collecting them next time.