Hi, Andrej!
Just a slightest correction: AS apartments are not within the Kopaonik center ( the place where the bus stops, or from where the ski lifts run).
They are in the so called "private-house-area" - Treska 4km from the ski center. But do not be disappointed with that fact, that area is fully reachable by buses, and is very comfortable.
See it yourself at http://www.asapartmani.com/ (there is an english version )
Have a nice time !