The owner should have said he needs more time but probably didn't want the confrontation. I would take it back immediately and tell him that you apologize if you rushed him and ask him if he can tune the bike better. You should also be able to take the bike back for a free tune-up once the parts have had time to wear in a little. Cables stretch, wheels get out of true and sometimes fasteners need tightening. That used to be around 30 days after purchase. Not sure what they do now though.