The only place I've seen RX original watercolors 
is Anime Museum. They went for around $900 on 
average. Keep checking them out. You can also try 
Mandarake Dailty Auctions.

As for sketches, from what I understand, the less 
light, the better. UV light will cause sketches 
to yellow over time. If kept in a binder or 
framed with conservation materials and displayed 
in a low lit area, I see no reason why they 
wouldn't last for decades before showing any 
signs of age.