There is not one particularly good time to buy a used boat. It would be smartest to buy a used boat when it is on sale and when you have the budget to buy the boat you want.
I should buy a Snowmobile, My boat sank when it ran into the ice on the lake last week.
In your dreams
There are pros and cons to both private and dealer boat purchases. The best price would probably be found with a private buy, as long as a buyer had the boat checked out thoroughly beforehand.
Some reasons a person would want to buy a boat is for fishing, waterskiing, for seclusion, inner tubing or to make a sales pitch. A boat can also be used to impress a girl.
Bass Boat Outlet is probably the best place to start looking. It's a used boat site. However, you will need to refine your search to the area you're looking to buy in. http://www.bassboatoutlet.com/
Buying a river boat is not so common and that is why it is hard to find a boat. If you really are interested in getting a river boat you should get a copy of How To Buy A Boat For Canal Or River from the Canal Boatbuilders Association.
The best time to buy a boat is in the winter.
Go to a boat dealer.
You should go to the boat supply store. You will find the best speakers for what you need and with installation on your boat with no compatibility issues.
I have used that site many times. Yes, it is.
Fx Sails offers quality and affordable sail boat sails. You can find them at: www.fxsails.com/. In your town, there should be marinas near where you launch your boat that will offer sail repair.