Here we have a nice Suntour XC-pro rear derailleur.
This was top of the line a long time ago and it shows very clearly when observing the mountain bike rear derailleur.
The front cage is made of aluminium, the back cage is steel. The pulleys are riding on ball bearings, they are still very smooth. The top pulley does not float like on a Shimano rear derailleur, it is fixed. Top part of rear mech is made of aluminium too, the rear nob that you adjust cable tension is made of brass I think (anyone know why?)
Condition of Suntour XC-Pro derailleur: In good mechanical condition.
One problem I noticed is that a plastic spacer have cracked between the cage and upper derailleur part. Since the part is made of plastic I am fairly sure anyone can glue a piece of plastic over the crack in order to reduce entrance of dust and other stuff.
Take a look at the picture so you can make up your mind!
Price 35 euro