I can only answer your first question: I think they're good for a really long time since they seem to be very well scealed. I have had mine for over 12 000 km so far and my mechanics was confident they didn't need anything special done to them. Keep in mind that this includes about 3000 km in the winter-time in BC.

To replace the bearings you have to send the wheel back to a Mavic facility I believe (not 100% sure - DebW or others will know better I'm sure).

This being said I don't know how much you'd get for it. I would assume they have lost more than 50% of their value. It also depends if you can sell them yourself or if you sell them through your bikeshop or something similar.