I do not think anyone is answering the question.
What needs to be done by previous owner is execute an "Application, Consent, And Approval For Withdrawal Of Application For Documentation Or Exchange Of Certificate Of Documentation (CG-4593)" http://www.uscg.mil/hq/cg5/nvdc/forms/cg4593.pdf
The issue is that when the intent of the new owner is to register the vessel in another country, most all countries have a "de-listing" methodology and form to be completed. Almost all countries insist on proof of "de-listing" before they will register...that way, you can only register a vessel in one country. The buyers agent should have insisted on this at closing...no form, no money
...then things become simple.
I think/believe that this form needs to be executed by the previous owner. However, if the previous owner executed the "SALE OR TRANSFER OF VESSEL" on the reverse of the US Certificate of Documentation, then the new owner can execute the above form and it will get the vessel removed from US documentation and satisfy the new country.
I hope this helps.
s/v BeBe, Amel 53