Maybe it is all the towels they have to replace: PurchasePro.com Inc. said late Thursday that it has signed an agreement with Hilton Hotels Corp. to build a secure, private purchasing portal for the 1,800-location resort chain. The deal is significant because PurchasePro, Las Vegas, will be responsible for managing more than $1.5 billion in annual purchases for Hilton. In addition to the private purchasing platform, PurchasePro plans to include Hilton in a trading exchange hub that will include other hotel chains. PurchasePro originally started as a buying hub for Las Vegas casinos. Beverly Hills, Calif.-based Hilton had been developing Internet buying in-house.