Getting new Car. Found a cheap standard 1996 Pontiac Sunfire!?

[~!Beautiful BC!~]

I was looking online and I found a standard 1996 Pontiac Sunfire for $500! We went and checked it out and test drove it and there is nothing majorly wrong that I could find or hear. There are little things wrong, like the trunk doesn't open and you have to slam the drivers door to shut it and stuff like that. But when we finished test driving it we parked the car and shut it off and my sister got into the car and was changing the gears and it started rolling forward when she didn't have the break down and she wasn't able to get it to catch a gear. We had to turn the car and put it in gear to get it to park properly. That is the only thing that has me worried. Is that something to take seriously? The guy said that has never happened before. The only reason he is selling it so cheap is because he is going to be gone by Monday night and can't bring the car. Other than that the car is in awesome shape. No weird noises or anything.