I have considered this and batted it around a lot. It is NOT GOOD to let any guests know in advance you are for sale. They will not stay with you, they will assume thread bare linens and deferred maintenance. As it told by so many guests we have had who experienced an inn for sale elsewhere...
I have had guests KNOW we are for sale and question me point blank. It is like we are committing some sort of sin, in their eyes. Like walking away from a happy marriage or something.
Don't do it.
Even when I want to post we are for sale, I have to be careful. In a for sale listing, I have had to ensure it DOES NOT LIST our B&B Name. They can click the link, they can know all about us, but not the name. Amazing how quickly a for sale listing can rise to the top of Google search results!
Sell to the people who are looking to buy, they will search the B&B for sale sites. Innsales.com is one I recommend, which goes to Aspirings via PAII and elsewhere. It has been revamped recently and has some great features for sellers and buyers!
http://innsales.com/.