You should offer the fair market value of a comparable boat.
A common issue is buyer fixation. The buyer decides one boat or one model is THE BOAT and he or she loses all objectivity. The seller then controls the transaction because the buyer will do pay or do anything to buy that particular boat or one of the limited supply of that particular model.
Why are you really buying a sailboat? What do you envision it will it do for you?
Whatever your reasons, there are likely plenty of comparable boats that would satisfy your purposes (that are not Sabre 34s). Offer what you believe you would have to pay to satisfy your boating needs.
Deciding on a Sabre complicates the process, because you will be paying for something beyond any real practical value for many sailors, for some element of owner satisfaction. Sabres are well-made, quality boats which display considerable craftsmanship, particularly in the interior. Racers describe these kinds of boats derogatorily as "furniture boats". Ultimately, this build quality and craftsmanship have little or nothing to do with getting from point A to point B, seaworthiness, cruising ability, sailing enjoyment, or sailing ability. To use a car analogy, you will be buying a Mercedes or a BMW, not a Toyota or a Honda, or even a Lexus or an Infiniti.
If you want a good value or a practical boat, there are any number of less expensive boats that will function just as well as a Sabre 34.
I don't begrudge anyone who wants to pay extra for the satisfaction of greater quality. If it satisfies your emotional needs, more power to you. Just understand why you are buying a particular type of boat and the costs that you pay for to satisfy that need.
I agree with making your offer more attractive by showing your ability to close the sale. Offer to provide the bank statement showing liquid funds in your account, or your loan pre-approval if you will be financing the purchase. Expect information from the seller about what you are purchasing.
As James said Sabres are quality build boats. That's why the 84 Sabre you are looking at has a higher resale value. not because its a furniture build boat. The 34 Sabre is also a good handling well balanced sailing boat. The accompaniments of having a great fit and finish inside make it a sought after brand. In addition it standard comes with oversized equipment and quality winches. The build quality is also why this boat has retained more of its % value than say a comparable year hunter/ bene. You tend to find more of the Sabres around % to the number built than many other boat models, again going back to the quality of build. Yes both sail, but some sail better,
. If you are using it for racing there is no value in the beautiful joinery, but your not so the joinery adds to the "feel" of the boat as a second place to be which is important to you. With Sabres you get the best of both worlds. A quick good sailing well designed boat with a superior wood finish inside.
As you look at making an offer as others have said there is no standard bid. In looking at the Waukeegan boat if that's the one I would say it looks in good condition from the pictures. It also doesn't have a lot of extra equipment on her. The way the sails are described ill bet the are original. The engine has very low hours for a 39 year old boat. If she has been kept on the Lakes her entire life it means she has been sailed and exposed lightly.
In negotiating first determine the price you will not go above in the end. What's its worth in the market and to you. This boat is listed at 49, way above its worth in the current market. Looking at comps I would say its 39-40is high end. Personally I would start at 32 and not go above 37. If there is apparently issues with the chain plates and have them fixed as a condition of sale or further reduction in price.
As James said don't get fixated. I would also continue looking and develop other potential boats so you don't feel pressured to not walk away.