It looks like Beltway power broker and friend-of-Bill (and Hillary) Liz Robbins has finally scored a buyer for her $15.5 million East Hampton beachfront estate. On the market since 2004, it was no slam-dunk to sell. The 5,000-square-foot shingled traditional, built in 1990, waited and waited for a suitor who’d appreciate its beach cabana and the proximity to Jerry Seinfeld and Larry Gagosian. (A broker at Allan Schneider had been marketing it, but Robbins and her husband, former Eyewitness News anchor Doug Johnson, recently turned to rival firm Prudential Douglas Elliman for an assist.) The buyer is said to be Wall Street securities trader and Houston Rockets owner Leslie Alexander. Robbins was traveling and unavailable; Alexander’s assistant said he had no comment.