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 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.The Trailer is to be sold free and clear of any liens, encumbrances, or mortgages. Motor Vehicle (DMV) Bill of Sale Form Affirms the information contained in a transaction of a vehicle for a payment amount. Illinois bill of sale forms are documents that provide information about the sale, purchase, or trade of property. Forms on this website are PDFs with fillable fields. Forms should be downloaded to your computer before filling in the fields. The buyer and seller should input their personal information and present identification to one another.