If they still work I always clean them up, it's the layer of carbon dust on it that kills them (I think).
If their dead then I usually give the owner 2 choices, £20 and ill just remove it, £(whatever a pcb costs) plus £20 and ill replace it. Never replaced one yet!
I replace them on sellers, but I've not had many with dead PCB's to sell. Had a hetty for 99p a year or so ago but gave that to Amys mum as she needed a vacuum, single speed and she didn't know it did anything else!