Well just a guess...
70/80s outboard values way below the holy grail 2000s last of the 15hp 2-strokes.
This made £300.. where does yours sit relative to that??
Evinrude 15 HP Outboard engine | eBay
If sold as non-running but believed to run and if it's a bit scuffed needs someone to take a chance around £100-£200.
If it's very tidy and you proved it started and ran fine then perhaps £300 or a bit more??
Edit: Having said all that I'd tidy it up, get running in a tub and chuck it on Ebay as this time of year they're a very popular size motor to sell for SIBs.... you might do far better than my ultra realistic figures.