This might not be as big a deal as you think. Yes, I know it's a huge PITA and disappointment, but it happens. IMO, he should have put more miles on the car before he delivered it to you. For something to happen that quick, the only thing that comes to mind is a failure to tighten something up properly or a failure to put the proper part on in the first place. When you say "smoke was coming out of the engine," can you be more specific? Front of the engine? Back by the firewall? Coming off the primary pipes? Did you happen to look at your gauges to see what your temperature and
oil pressure were? Hang in there. The nice thing about these cars is that they're basically pretty easy to fix once you figure out what's broken. And when you have a really bad noise and smoke, it's usually pretty easy to figure it out.