Andrew Rollert had a dream: a mobile online parking marketplace. There are 21,700 metered parking places in Manhattan, which are all pretty much always occupied. His idea was to let parkers sell the location of their spots as they leave, and a map would lead the buyer to the promised land; an eBay-like user rating system would “weed out the jerks.” His company, SpotScout, would take a 15-percent cut. To achieve his dream, Rollert applied for a patent, courted investors, hired computer programmers. The media, from The Wall Street Journal to NPR, hyped it. Locally, 165,000 drivers preregistered. The only problem: the sale of public spaces is illegal. Rollert insists, “We’re not selling spaces. We’re selling the information about the parking spaces.” But a DOT spokesman doesn’t buy that: “It’s also illegal to block, save, or reserve public parking spots.” For now, Rollert’s launching a marketplace for garage spaces and private parking spots on September 18, and consulting with lawyers. Later he plans to attempt to tempt the city, perhaps with free foliage. “We might, for example, take a small piece of the transaction fee and put it into a fund for trees.” He has yet to approach the city with the idea.