There are a couple options to consider at this point. Do you want the truck back or do you want the money owed from the buyer? Return of the truck is the simplest course of action, if it is in reasonable shape (not totaled or damaged). I would let the towing company know the situation. They are able to release the truck to the registered owner but they do require their fees to be paid. You can attempt to recover the towing fees from the buyer but if he is not willing or able to pay for the truck, recovering the fees can be difficult. If the truck is damaged to the point where the value is less than $700 then let the towing company have the truck. Alternatively, you can take the buyer to small claims court for the balance owed on the truck. But again enforcing the judgement and collecting the money is an issue. Do you know if the buyer has assets or an employer, so you might have a way to collect on a judgement?
Oregon law requirews that a seller of a vehicle report the sale within 10 days of the sale; see: https://www.oregondmv.com/Online/Sell.htm It would appear you are receivng the notice from the towing compay either because you did not report the sale, or you sold the truck and put your name on the title as the legal owner to secure your interest. In either event, you should speak with an atty to determine just what you rights are or the truck will be sold and your recourse will be to sue the buyer who did not pay you and who is not likely to pay you now. The lawyer may be able to help there too.