An Authority Site that's been actively developed and marketed for a year or so is a valuable internet property. An immeasurable amount of time, expertise, creativity, linking, satellite content development, and other resources have gone into that site to get it where it is.
Many individuals and companies would rather pay top dollar for a successful existing site than build one completely from scratch.
If you enjoy developing Authority Sites, you may even want to consider developing and selling them as a business model.
And even if that's not your plan, you may be approached by someone who's interested in buying your site - something you'll have to consider carefully and possibly negotiate back and forth before reaching a decision.
If selling your site is something you're considering, or if you've been approached by a potential buyer, read this thread from the ASC forum: