Here's a link to the "Applicable Federal
Rates" for 2012.
The minimum short term rates are so low, ie. a fraction of 1% per annum, that almost any rate you want to charge would be sufficient.
Do you have a rate in mind?
What is your objective?
You might consider giving your friends an option or a first refusal, but to tie up your house for an unknown period of time, in exchange for such a small amount of compensation
, would be a very favorable deal for them, but not for you.
What if their house doesn't sell? At a minimum, you should attach a time limit to the deal. Also, if you don't have any security for the note, situations change & there would be no incentive for them to complete the deal, except your friendship.