If the buyer or assignee of the bike isn't a resident of a U.S. State and lives outside of the United States, an option is to do up an Affidavit of Title and a Notice of Sale and serving copies of the documents to the AG or DMV types of the state associated with the previous purchaser and of course clearly evidencing a non-resident address. A USDOT # with "no cargo", "private property", "not-commercial", "exempt" entries can be obtained to go along with that. Alternatively, if the assignee or buyer has some kind of exemption where they live they might be able to acquire plates according to that exemption.
If they are not bonded or if they are oath with unsightly reservation, then that would be an imbalanced contract--perhaps even one-sided--all consideration on your side, none on theirs. It should be easy enough to do a general inquest in a county that says to anyone holding office with any reservation whatsoever or who has failed to file and take the required oath that they either GTFO or be bound by the oath from the inception of their office without any reservation whatsoever + case jacket + certificates of service + certificates of default and just see who stays and who doesn't--maybe giving them 15 days. Seems quite worthwhile as to effort if it means keeping your county clean.
Also, if the oath of office looks sketchy, serve them up with a notice that it seems ??? and that it shall be from the date of your notice and retroactively construed that they meant {what the required oath says and without any funny capitalization} + case jacket + certificate of service + etc + reminded that they are bound to that oath as it is required and that your correction or clarification supersedes and replaces their ??? filing.