Dear Pelican Bob,
What the boat is worth depends on its age (list the full Hull ID number from the upper right hand side of the transom or a serial number from a plate on the deck near the mast hole) general condition of hull, spars, rudder/tiller, dagger board, sail and lines, weight of hull (130 lbs is the average new boat weight - a heavy boat is worth less than a minimum weight boat) and extras (serviceable trailer, beach dolly, life jackets etc.) Is there any damage that needs repair? Reply with the full Hull ID number and the rest of the information and I or others can give you a ball park price. Big question(s): Does the boat has a storage compartment aft of the cockpit and what kind of rudder fittings does it have, i.e. are there bronze rudder fittings on the keel and deck with a wing nut adjuster bolt or does the hull have a stainless steel "U" shaped fitting bolted on the stern along with a rudder with springs in the side of the upper rudder assembly?
Selling it on this website is free and effective - Ebay is another option. Donating it is always a good option (but you will need an appraisal for tax purposes.)
Like Al said price depends much on condition. Market plays a role too so I don't think there a universal price scale. M75 is a 1975 model. Again that doesn't tell it's worth. Value is a combination of condition, supply & demand, buyer emotion, and your presentation. Newer boats are more consistantly priced. Any of these variables could sway the price by a couple hundred for older ones like yours. I see recent prices ranging from $175 to $899 depending on all of the above.
If I recall it right the IRS allows up to $800 in charitable contributions without an appraisal or original receipt. Just be prepared with a list of comparable used boat prices from the newspaper to justify whatever you claim.