What makes art valuable

Digital fine-art list
Arts & crafts

These are the factors that I think influence people when buying art. I've listed them in descending order of importance.

Artist Who did it? (A small cheap pencil-drawing found at a second-hand shop by Van Gogh when he was 10 will be perceived very valuable only because of the artist)
Exclusivity How many exist? Is it a one-off? 10s, 100s, 1000s or more?
Direct contact Did the artist have direct contact with the paper (e.g., did they apply paint to it) or in the printing process (hand-made wood block from which a unique print was made)
Size How big is it? (The bigger the better.)
Medium Oil, pastel, watercolour, acrylic, pencil, digital
Effort How long did it take to make? Five seconds, five years?
Skill How skilled the artist is in the chosen medium
Venue Where it's being sold - an art gallery or an outdoor fair
Price How much is it? Higher the price, higher the perceived value
Rarity Is the artist still alive and producing works or is this one of a limited set of works?
Artistic merit Is it a good picture?