If you sell or trade in during fall, you avoid both the cost of winter storage and the cost of winterizing your engine(s) and onboard water systems.
If you wait to sell or trade-in your boat in spring, it will technically be a year older, and thus, worth less. Plus, price increases on new boats will likely have taken effect.
Dealers often have great deals on "leftover" boats in the fall; and with showroom traffic winding down, a dealer's willingness to haggle may be increased. Selling or trading your current boat in fall, frees you up to take advantage of those opportunities.
Fall boat shows often offer solid boat and marine engine manufacturer incentives that those divested of a boat can take advantage of. If you are still saddled with your current boat, you may not be in a position to ( or have the desire to) purchase a boat and get these deals.
If you buy a new --or new to you--boat in fall, you'll be ready to chase down fish, swerve through a slalom course, wakesurf or simply enjoy the sunset afloat as soon as the weather breaks in spring.