Think skateboard trucks and wheels - with a kickplate
to hide them and 1/16 inch shorter than the distance
between the bottom of the wheels and the top of the
truck. Non-marring, good weight bearing support,
readily available in most towns (or in your kid's room)
roll smoothly because of composition of the wheels
and because they use bearings ...
BUT - be very aware that the door bottom must
accurately parallel the floor throughout it's
range of motion or the wheels with either try
to lift the door - screwing up the hinges OR
not make contact with the floor unless the door
sags - also not good.