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.Prepare a bill of sale for the buyer. Include the date of sale and the price paid. Although some states require a bill of sale during the transaction, it is entirely optional in Illinois. This is the motor number or serial number required for identification purposes. The Electronic Registration and Title (ERT) System allows you to complete and print an Application for Vehicle Transaction(s) (VSD 190) online.