Here's a few choices:
A) Save up some $ for that key. Some people would sell most of their collections to be able to afford their dream book.
B) Wait for a buying opportunity when the market is not so hot. It's cyclical and there will be deals to be had for sure.
C) Buy a reprint of that key book if all else fails, which practically looks almost identical to the real thing in some cases. I'd rather have something tangible to flip through than "own" an image of a book which anyone can just look at for free. Also, what if that person has a stake ownership and never wants to sell, but majority does than that person would be forced to sell their share(s).