It would be helpful to know what state you are in.

I am in California. I feel that you entered into an agreement that prohibits you from selling their property at auction. Your entering into that agreement is not wrong, it's just that you need to go after them in a civil court. If you can use the small claims court system for this debt, it would not be very expensive to pursue them.

We have to recognize that once we accept payment, the lien process must start over. Also, it is wise to accept that payment as payment in full and require that they vacate.

You probably got more money out of them then you would have in an auction. Your owner, hopefully, is the winner. What you do now that they are still there is to make them an offer they can't refuse and require them to vacate.

I worked for a bank in criminal investigation. I learned that it's sometimes better to write off your loss than to spend a great deal of energy pursuing someone who doesn't have the money,,,even if you won a lawsuit, they couldn't pay, or spending time with an attorney (sorry, attorneys, I value you, but it does become expensive seeking legal advice so we have to weight that expense against the possible loss exposure.)