Take it for face value. Average retail is just that, average. What I would look at is overall condition of the hull, motor, and trailer in the average retail price. Don't bother with the other filters. Then, find out from the seller about the age of the add ons. Trolling motor, electronics, power poles, batteries, trailer tires etc. If they are 3 years old or less, I'd add 50% of the cost of those items on to the price. So, if it has a $3k Ultrex, I'd figure $1500 on top of the average retail. $1500 graph, add $750. and so on. If the add ons are more than 3 years old, I'd be hesitant to add much onto the retail price unless the condition is immaculate.
Buying a 25 year old boat, I'd make a deal contingent on a shop giving the boat/motor a once over and split the cost with the seller. I'd much rather be out $150 knowing ahead of time the boat was a dud, then out 7 grand and being stuck with a problem child.