Andrew Youderian wrote an incredibly thorough post on everything you should consider when selling your business. It's specific to eCommerce businesses — as he has sold two of them — but it's applicable to pretty much anyone that runs a business, I think.
I had an awesome chat with Andrew today. He runs eCommerce Fuel — a community much like Post Status, but specifically for high-revenue eCommerce store owners. If you or any of your clients run good size eCommerce shops, it's a good site (and podcast) to dig into.