Timing the sale of a business makes sense in some circumstances, but not in all. Selling a business in an up market means an owner will likely be investing the sale proceeds in an up market as well. Just like selling your house in an up market, should you need to continue living in the same city, you will be paying a higher price for a home, it only makes sense if you are leaving the region such as moving from Vancouver to Penticton for example.
Selling when the business is doing well, with all the Value Drivers operating at peak effectiveness, makes way more sense. Focusing on building a solid company, ensuring that you spend time to ensure the Value Drivers are optimized will realize greater value regardless of what the market looks like - a good business will always attract a good buyer.