Does Restoring an Item Ruin its Value?

If you want to make some money, buy the anniversary sets (i.e. the metallic James, Percy, Nia and Thomas).  They may worth a bit down the road.  

Replacing the old stickers on tenders with new ones won't affect the value.   if you restoring the engine by lets say spray paint, the value will go down significantly.  It really depends on what type of restoration you want to do.