your offer should be based on what the boat is worth to you...without professional help (a broker) you have little idea what the sold boats actually sold for...the boat is worth exactly what some one is willing to pay for it...you may or may not get a "deal"...if that is your goal, I would be very leery..."deals" often turn out costing way more than a correctly priced, correctly maintained boat in the long run, or even short run. YMMV.
that being said, most buyers will make an offer - subject to:
1. acceptable financing to you
2. Successful Survey
3. Successful sea trial
4. Rigging survey (if desired)
5. Engine survey (if desired)
6. Anything else you may want to add here...is where you do it, not after the offer is presented..
A contract is written and presented, along with your deposit money (again this is where the broker is usually of value, if no broker find a local bank or use BoatUS) and then you proceed to go through the list - at YOUR expense unless agreed to previously.
As you satisfy the contingencies, you move closer to a decision and lose leverage to back out of the deal if you find things are not as they appear.
When the survey has been completed by a competent surveyor YOU have hired (not last years paperwork, or a friend or a surveyor identified by the seller or his agent), you often can re-negotiate your initial offer i.e. - have the seller reduce his price, have the seller "fix" those items, or simply walk away and have your deposit returned. You will be out the haul/survey charges. And the seller may or may not respond as you would like. This is generally "big" things, show stoppers and the like. Not nuisance things.
I would read up here and at other sailing forums for more details. There are volumes written on "value, offers, deals, soldboats.com, fair market value, what it is really worth", etc..
Best of luck