Can you use a 12 foot trailer as a car hauler?

You can but you may have a fitting problem. The average mid-size car is around 16 feet long. The other issue you may have is will your 12 foot trailer handle the 3500 lbs of weight. If your trailer is a single axle make sure your insurance is paid in full before you try. If you miss the weight distribution and get it tail heavy you will find out quickly what a 4,000 plus pound whip feels like. I have witnessed a similar set up turn a dually over like it was a tonka toy.

Now if you think the car will fit and the trailer will handle the weight the final step is to look at the angle of the incline from whatever ramp system you plan to use while loading. If the angle is steep or the rear of trailer is higher than the front of trailer the car will hang when it tries to make the transition from ramp to trailer bed.