There are no NPC'S to interact with to buy things

[lispi]

dtgerger

 

[Nalurah]

True, there are signs in the shops.

 

[SI Ali Barber]

We use Panels / Signs that are interactable, they have the RPUK logo on the front of them if you are getting confused.

This image: 

 

[Roberts]

I am pretty sure the reason we don't use NPC's is to reduce the lag it causes on the server. 
As stated above you can use the signs provided at the points of interest.