What would be a fair selling price for a complete set of bound volumes
of the scientific journal "Nature", comprising all issues from 1869 to the present?

Put the set up for sale on e-bay with a reserve price you would be
ecstatic with; if the set sells for your reserve, you will be happy;
it may go up much higher than that; and if it doesn't meet your
reserve at least you will have an idea of what the public values the
set for.

If the set you are talking about is one you want to buy, it is just a
matter of what the set is worth the to you.