If you need to go to a vehicle shop but you do not know where to go because you are very lost, then you should check out the vehicle shop in Florida called Ben's Repair Shop.

Go to the Auto shop

Yes, a repair shop can place a lien on a vehicle for unpaid repair/maintenance work. If the vehicle is taken to that shop, they can hold that vehicle until the bills are paid, even if you are not the owner who originally incurred the charges.

Go to shop then click Pixel shop then click automobile and then buy a car

There is nowhere in GTA IV that you can purchase a vehicle.