I'm not sure what they did was illegal- Don't know IL law.
GA titles say on them, "TO BE COMPLETED BY SELLER..." That would be the previous owner, not the shop, who failed to complete it. The shop was never the owner.
However, if the shop had the right to function as an agent of the PO and the date of sale or transfer (filled in by them) was the date YOU purchased it, I think the sale was officially complete on that day, and you bought it from the PO, not the shop (assuming that the title was completed by the shop on behalf of the PO the day you bought it). They were just acting as an agent of the owner.
That's not such a stretch legally. The PO likely had a debt with the shop, they received the car on consignment and sold it on behalf of the PO, accepting the purchase price as payment of his debt to them.
Leave the lawyers out of this. No case.
