Ideas on promoting economy (trading and interaction between players):

Add weight based on items in the inventory that will slow a player down
This promotes the use of rail/vehicle transport. Due to the normally costly nature of these systems some players should start making them and hopefully have their services rented
Potential problem with vandalism

Automation:
Specifically automated mining basically allows people to do everything, why trade when you can easily DIY?

Teleportation:
Easy mode workaround to transportation

Closed vs. Open economy:
Typically an open economy (infinite source of some description) creates inflation because money's value changes (basic supply vs demand), a closed economy avoids this because the supply stays the same but this can do the opposite effect as an open economy. To combat this you do need a way of introducing more money (maybe give players X starting currency/?).
An open economy works if there is a gold sink to take money out of the market

Safe trading:
To help combat scamming players need a way to either:
Trade in a safe way (a GUI type thing is a good way to do this) or something along the lines of a reputation system (players with negative reputation could be picked up by the cops GMOD RP style)