Dealers will do a deal on bikes to try and make sales. Do they also reduce the cost of apparel, helmets clothing too? Can't say I've ever been in a dealership in winter. And would online retailers do the same?
In all honesty I've never seen any substantial reduction in prices for bikes or clobber over winter. Dealers needing to sell in numbers will of course offer incentives (nothing substantial though), but as a private buyer/seller the weather doesn't seem to make much difference at all. I've certainly had no problems selling bikes in the winter without having to cut prices.