The best money you can spend before selling would be on a full detail by or yourself a qualified marine detailer.
I would focus on the following items in the detail.
#1 Hull - Needs to be shiny and as close to new as is humanly possible. Compounding & polishing or one of the acrylics like Poli-Glow are huge pluses.
#2 Boot stripe - Must be cleaned of any tannin or rust staining if it is painted and showing it's age paint it.
#3 Canvas - Glass should be clear and the canvas should be repelling water 303 Fabric Guard will re-waterproof and Imar Strataglass cleaner is a great product for the glass.
#4 Boat bottom - Prop should be clean as should the shaft and have a new zinc thrown on. If on the hard it should be prepped and ready for pain or already painted if you have been using a multi-season ablative. Little details like this make can make a big impression.
#5 Deck - Should be clean, sooth areas should be waxed with a produt designed for waxing decks like Woody Wax.
#6 Metal - Should be shiny and free of rust spots
#7 Portlights / Hatches - Should also be shiny and a good plastic polish will achieve this.
#8 Interior - Every nook and cranny should be cleaned of any mold or dirt & grime and it should smell fresh and clean.
#9 Bilge - It should be clean and eat out of it spotless! This is a huge thing for many buyers as a clean bilge has been known to represent a well cared for boat. It is one of the first things many experienced buyers look when entering a boat.
#10 Engine / Engine Room - Should also be spotless showing no oil drips or fan belt residue. Any rust should be coated with Boeshield or similar to make it less "offensive". If rust is real bad it should be painted with Ospho or similar then matching color engine paint.
#11 Ice Box - Clean and NO mold with hinges and latches working.
#12 Lines & Running Rigging - If dirty they should be washed in a front loader with Ivory Snow, Dreft or other non-detergent cleaners and a very small amount of bleach. This can be done easily in a mesh bag.
#13 Engine Oil - Must be clean and recently changed with a record book showing maintenance records.
#14 Maintenance Records / Manuals - Should be available and in a three ring binder for inspection. It is a good idea to display these on the nav table or the salon table.
#15 Lockers & Stowage - Remove anything you are not selling with the boat and organize every locker & cabinet to show & display maximum storage space. Get rid of any personal clutter.
#16 Personal Items - Remove any personal efects not transferring with the boat.
#17 Spares - Organize any spare parts or parts transferring with the sale and organize and label them.
#18 Electrical - Everything should work and you should test everyhthing to prevent surprises. Any loose or sloppy wires shoul be bundled and neatend up with zip ties.
#19 Sails - Sails should be available for inspection but not on board. They clutter up the interior and signifigant others can have a tough time seeing past this clutter.
#20 Upgrades List - A complete list of every upgrade, or maintenance item, and approximate dates should be sitting on the galley counter or chart table.
#21 Screw holes - Cover them, hide them or put a scew in them it looks better than an empty screw hole.
#22 Seacocks & Hoses - All hoses should be visuallu inspected and wiped clean. ArmourAll does amazing things to dirty old hoses. Hose clamps should be clean and rust free. Seacocks should be labeled and open and close freely.
#23 Anchor Locker - Should be clean and the rode should also be washed if nylon. A dirty muddy chain and rode does not convey confidence in upkeep & maintenance.
#24 Lifelines - Should be cleaned, Magic Erasers work great for this, and any signs of rust should be cleaned.
#25 Wood - Exterior wood should be touched up if varnished or if real bad stripped and treated with teak oil. Teak oil takes very little effort to do and makes a HUGE differece when comparedd to peeling varnish or gray weathered teak. Interior wood should be touched up if varnished and then treated with a product like
Orange Glo (LINK). If the finish is oiled it should be cleaned and then re-oiled with a product like
Weimans Lemon Oil (LINK). Neither Weiman's or Orange Glo contain any silicone and are good silicone free choices!
By spending the relatively minimal funds to get the above things done it will return much more money than doing nothing. Unfortunately, or fortunately for owners that do this, most owners never take the time to make their vessel ready for sale or staged. This stuff is easy to do! Is a day or two of your time worth another 2, 4, 6, 10 or even 20k more in re-sale value?? Clean boats sell and they sell for more money and faster this is why it is always nearly impossible to find one!!
You only get the opportunity to make a first impression once!