I recently bought a page of art from a book that has exactly ZERO sales comps. The only thing I could find was a page from that book on a dealer website, and the price the dealer was asking. I thought my page was better (the dealer page was a full page splash, but not as interesting), and I paid less than what the Dealer was asking. And, frankly, I have no idea how long the dealer had that page sitting there at the higher price unsold (probably for a while), so my price was probably more accurate for my page.
I'm getting relatively good at guesstimating value. I tend to undervalue things, though.