Dealing Doppelganger

Publisher's blurb: This is the 65th in an ongoing series aimed to provide the overworked DM with ready-to run encounters. The PCs come across a doppelganger who has the best deals in the wild. Will the PCs engage in peaceful commerce or will they decide to get the 100% murder discount on goods? The dealing doppelganger is CR 3; but this need not be a combat encounter.