Rafting products cannot be compared on a standard inflationary scale. It's a niche market, not typical goods and services. Niche products in the outdoor community, especially whitewater sports has nearly doubled in the last 7-10 years.
An Aire raft that was sub $3K 10-12 years ago would now cost $5-6K now. (not considering special sales or the like) A premium brand raft, frame, oars, straps and a few basics just to get running day trips could easily run $8-9K. Add in "stuff" for overnighters and you could hit $10-12K for all new items.
If you got a straight 10% off a "basics" package, you're still looking at $7500 new, no frills. If a person who has this "premium" equipment and took great care of it, for sale at say $5500, that's about 35% off retail. That doesn't seem out of line to me for a package deal that someone took great care of.
Miles don't depreciate this type of equipment like it does an automobile. It's time and care.