Organising Paperwork
To obtain top price for your boat, you should ensure that all you boat documents are in order.
Documents of compliance
If your boat was manufactured after June 16th 1998 you should have been given a Declaration of Conformity, stating that your boat complies with the EU Recreational Craft Directive (RCD). If you've lost this piece of paper, there should be the Boat Builder's CE plate inside your boat; the original manufacturer will be shown on the plate, contact them for a duplicate Declaration.
If your boat does not conform to the RCD regulations because, for example, you bought it as an American import, you should consider obtaining a CE mark. You might find that the cost of obtaining one is much less than the amount that you can add to the sales price for your boat.
Additionally, you should have your boat's original 'Sales Invoice', that shows that the VAT on your boat was accounted for. Once more, if you have lost this, contact the Boat Builder. If they didn't sell the boat directly themselves, they should know who did, if only from their warranty records. When you know who sold the boat originally, you can contact them for a copy of the original 'Sales Invoice'.
It's important that you have this document. Any prospective buyer seeking to borrow money to buy your boat will probably only be able to do so, if the lender has a sight of this bill.
Service and Maintenance
Hopefully, you'll also have in your possession the boat's original operational manual together with copies of invoices showing the engine's service history and replacement dates of any key components.
The more of these records that you possess the more that you can justify a top price for your boat.
Licences
Finally, as far as paperwork goes, you should check that any relevant licences are current and not out of date.
If you own a canal boat check the date of you Boat Safety Certificate. If it's out of date, renew it before you put your boat up for sale.
If your boat is intended for use on the Inland Waterways, check that you have a current British Waterways Board licence. Again, if it's out of date, renew it before you put your boat up for sale.
Alternatively, if your boat has a fixed VHF radio, check and ensure that the radio licence is current.