Most delays in a yacht sale come from an incomplete file, discovered only once negotiations are underway. It is best to gather in advance the registration or ownership documents, the maintenance logbook, invoices for major work, and compliance certificates (CE, tonnage).
A clear history of the equipment (engines, electronics, rigging) and any declared incidents also builds buyer confidence and limits questions during negotiation. For older boats, a recent survey report can make the difference against competing offers.
Finally, anticipating the tax question (VAT status, flag) avoids unpleasant surprises with the notary or authorities when finalising the sale.