Auction season in Manhattan is a two-week spending spree of paddle-waving rich people and art dealers in Prada suits, all of them vying for highbrow booty at Christie’s and its archrival, Sotheby’s. The regulars were asking questions like “How much will the Hopper fetch?” and “Which house will gross more?” But if you’d never visited Planet Expensive Art, you didn’t care about that, not after you spotted those Friedmans. After that, all you could wonder is: How does an artist peddle his doody, not to mention his doodle? And here’s another stumper: Who would buy it?