Sold car buyer never transfered title

Hello everyone just like many others I am in a weird situation. I sold a car in CO that was titled and registered in CA. Buyer unfortunately never did his part and transferred title to his name. Luckily for me I submitted my release of liability the date of sale. Well now, I have the Colorado toll roads billing me and I’ve explained to them multiple times I sold the car in Sept of 2025 and they need a notarized bill of sale to honor it on their end, which I do not have. I’ve called CA dmv multiple times and they’re not much help either, they say only say on their computer it’s documented I have sold car from release of liability but on vehicle registration history it states I am still the registered owner. A few days ago CO toll roads sent the bill to collections and I received mail from an agency for a car that I haven’t owned since sept of 2025. Does anyone have any insight on how I should handle this? I sold the car to a guy on offer up and don’t have his phone number. I only have his name and whatever address he wrote on the release of liability.