“He draws money […] He doesn’t sell his drawings. That’s rule number one. Instead he ‘spends them. That’s rule number two. So far, he’s spent about $250,000 worth of Boggs’ bills. […] Once Boggs has completed a transaction, he keeps mum about it for twenty-four hours, so the person who got the drawing has ‘time, unbothered, to think about what’s just transpired.’ That’s rule number three. After that—rule number four—Boggs will sell the receipt and change from the transaction to a collector. Via the receipt, the collector can track down the owner of the Boggs bill and negotiate a deal ‘so as to complete the work.’” (source) More here and here. (pointed out to me by Richard and Seth - thanks!)