Registering a trailer in Illinois does not require an Illinois trailer bill of sale, but you should have one when you sell or buy a trailer. An Illinois trailer bill of sale form distinguishes the buyer from the seller in a trade deal.Prepare a bill of sale for the buyer. Include the date of sale and the price paid. An Illinois motor vehicle bill of sale documents the purchase and sale of a motor vehicle between a private seller and buyer in the State of Illinois. A WI bill of sale should get you an IL title.