Rule of thumb for outboards is about $150 per HP for a 4-stroke, $100 per HP for a 2-stroke, showroom new with warranty. Some manufacturers are a bit higher than that (Mercury, Honda) while others are quite close.
Adjust your used price based on a discount from those figures, based on condition, hours, etc.
An outboard is considered end-of-life at 2,000 hours. You may well get more hours out of it, especially if well maintained and used occasionally, but by 2,000 about 50% have failed or are on the verge of failure.
I highly recommend running the motor in a drum of water before buying. Beware lower unit damage ... bent prop-shaft is common in locations where a motor can be expected to hit underwater obstructions; a new lower unit is rarely less than $2,000.
Outboard Motor power ratings are regulated; the advertised power cannot be more than 10% higher or lower than actual power at the prop housing. "9.9" HP motors exist because many lakes limit gas powered engines to "less than 10 HP" but almost without exception a 9.9 is 10~10.75 HP