Expired registration does not prevent you from selling your vehicle to EZ Cash Cars. Registration status affects the legality of driving the vehicle on public roads, not the underlying ownership rights or your ability to transfer that ownership to a buyer.
The title is the document that establishes legal ownership in Illinois. As long as you have the title in your name (or can obtain a duplicate title), expired registration is not a barrier to the sale. You do not need to renew the registration before calling EZ Cash Cars.
One thing to note: do not renew the registration on a vehicle you are planning to sell to a junk car buyer. That cost will not be recovered from the sale price and is simply wasted money. Call EZ Cash Cars first, confirm the sale plan, and proceed directly without any additional registration investment.