Per all the "trying to help you" posts above, you should check out all the basics; air, ignition, fuel and driveline, then if you still have a problem, take it to a knowledgeable mechanic. Any advice you get here is fundamental that you should be able to do yourself, and if not, you should immediately find professional help. Kind hearted as these posts are, you are only frustrating the readers since they cannot approach your bike and put an eye and a hand on the bike to check it out. Good luck.
Too late, but though sellers bitch about demo rides, there always comes a time that the buyer has the right to test out the merchandise. If that doesn't work for the seller, most would leave the bike where it sat.