Although businesses sell year-round, I find business opportunities sales stats hit their highest marks during the fall and winter. There are some businesses that sell easier during certain times of the year, for instance a tanning salon will sell faster during winter and fall for obvious reason, because of daylight savings times, etc. Businesses that are in coastal areas, especially on a boardwalk or peninsula tend to have premium rents and so when their monthly gross sales are down it makes them slighter harder to sell. If you buy a business in their "off market" you can sometimes get it at a lower price.
It's important to remember that selling a business can take on average 4 months to sell and then 45-60 days of escrow. For business buyers, it's important to look at the monthly gross sales of the business for the entire year. I have sold businesses in places like Laguna Beach, California where they are very much dependent on foot traffic, and so during rainy seasons when people tend to go to the beach less, their monthly sales have a large drop off, but their expensive rents stay the same.
I'd be curious to hear from others on their opinion of market trends during the year.