NADA is a good source for looking up prices, but I would caution that the prices they post may be more skewed to what a "reseller" would pay as opposed to a consumer.
If I was a "marine dealer" I would need to make some money on a used sale, plus I have to assume that I may need to do some maintenance before the motor would be ready to sell.
So to give you an example, I "know" that a mid/late 1990's 9.9 two stroke Merc, "in my area" (and the location is important when figuring price) will sell for about a grand.
Now if I am going to buy one to resell I may want to make $200 "profit". I also have to assume that it will need waterpump service, a couple new spark plugs, maybe a carb kit etc before it's up to the standard that I need it to be to resell. So if I estimate that I may need $200 worth of parts/service, plus the $200 I want to make, that "thousand dollar motor" is worth not a penny more than $600 "to me". And if there are other potential issues (chipped prop etc), that price "that I will pay" drops further.
And that's why NADA gives a range - almost no work required to use = higher price etc.
Now, you are talking about a 4 stroke here. Maintenance over it's lifetime (and this one is "almost" 17 model years old) is far more important than it was on older 2 strokes.
Regardless of what you determine to be a "fair price" I absolutely would require that it be "checked" by a certified Merc Tech as a condition of the sale. You don't want to buy a can of worms - regardless of "how pretty" it looks on the outside, the cost of servicing a 4 stroke can quickly add up to the cost of a new motor if there is a significant internal problem.
If all checks out I personally see the value of a used outboard, in the portable class, to be somewhere in the 50 to 70% range of the cost of the same thing "brand new".
So if you can get a brand new 15 horse, with similar options, for say $2200 then I would be looking to spend perhaps "no more than" $1400'ish if it checked out "perfectly" and needed "nothing" to use it tomorrow. Any potential "rehab" costs would get deducted from that price until the price was viable. And if you can't do the work yourself you have to factor that you may pay upwards of $100/hour, plus parts, to get it "ready to go" (which is why a tech check is absolutely required, complete with estimate).