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Short answer: You need to sea trial the boat and make sure that it will run at in 5,200 to 5,500 rpm range with the throttle fully open. If the current owner won't allow that / the engine can't achieve it, it might be time to look at other boats.

Longer answer: Your quoted question above is not a question we can answer without a lot more information. However, note that we keep going back to what the engine RPM is at full throttle. It's not that we all go flying around at wide open throttle all the time, it's pretty rare that conditions allow that in a 17 foot open boat. It has to do with set up / running conditions the engine has experienced over its life, roughly 28 years in the case of the boat you're looking at.

Propeller pitch can be somewhat compared to gear ratios on a car; the higher the pitch, the further the theoretical movement through the water for each revolution. For example, with 0 slip, a 21 inch pitch prop would move the boat 21 inches forward per turn. There's no such thing as 0 slip by the way, as nothing is 100% efficient.

Too much pitch will "lug" the engine similar to driving a stick shift car up a steep hill in too high a gear. Too little pitch might cause the engine to rev too high, similar to driving a stick shift car at highway speeds in one of the lower gears.

Outboards need to run within an RPM range based on the manufacturers design; the 1989 Mercury on that boat needs to run at the speeds mentioned above at full throttle. Many years of operation with too much prop pitch will damage the crank and rod bearings and can cause them too fail. Not to open a separate can of worms, there are other mechanical issues that can keep an engine from running at full speed as others have mentioned above.

You don't want to buy someone else's problem, if you have your heart set on this boat it may be worthwhile to pay a Mercury tech to inspect the engine. Replacement engines are not inexpensive, trust me... Good luck!

Edit: I've seen a couple of your other posts where you mention price and a couple of pictures. Looks like a well kept boat and a pretty decent price for that boat here in the northeast. If it's in really good shape, the engine may have low hours. Maybe the current owner is a "collector" or a little quirky in the way he runs the boat to explain the 75% throttle statement. Again, best of luck.