I've sold all my previous boats for more than I bought them. Once the boat is 30+ years old, price depends a lot on condition.
For example, I bought my second boat in some dude's backyard, in the middle of some major renovations the guy was too old to finish. I wrapped up all the critical stuff, sailed on it for 2 seasons with my family, and sold it in the water at a marina (meaning it was at least navigable; it wasn't when I bought it). If you only look at purchase price, I almost doubled it, but when you account for everything I spent to make it navigable, I'm probably close to even (especially if you count my labor).
For example, I bought my second boat in some dude's backyard, in the middle of some major renovations the guy was too old to finish. I wrapped up all the critical stuff, sailed on it for 2 seasons with my family, and sold it in the water at a marina (meaning it was at least navigable; it wasn't when I bought it). If you only look at purchase price, I almost doubled it, but when you account for everything I spent to make it navigable, I'm probably close to even (especially if you count my labor).