Look at the capacitors (especially the big green ones ), in case you see any leak - you must replace it.

I am through process of cleaning and lubricating of the push buttons. The

Balance-, Bass- and Treble-controlls need Deoxit, the turntable and speakers seems ok. 

Is there something more which I should look into, with my limited knowledge in "electronics", before I put it for sale?

(the TR2055 will have its own topic)