Not legally, no. But it would be nice of you. Unless the terms of sale will include keeping all current employees. Rotten to hire someone, and not tell them they could lose their job when the business is sold soon.
I do not think you need to tell a prospective employer that your business is for sale. I believe this is because the prospective employer is not an actual employer yet and the dealings of the company are not his/her concern. You can consult with a layer to be sure and to know how to go about it.
If you were a good and caring person you sure would tell a prospective employee about the sale of the company. What if this person was choosing between two jobs? It is not a happy surprise after an investment.
You don't have to tell them. Selling a company does not mean the loss of jobs. On the other hand telling them could mean a loss of employees which would have an impact of the sale of the company. I have built and sold two companies with no job loss except mine.