I have done it twice. The second one went smoothly, I foreclosed on the first and went back into the business for another 20 years. 

Look for a brokerage firm that specializes in selling tax businesses in your area. 

https://www.google.com/search?q=tax+practice+sale

https://www.google.com/search?q=accounting+practice+sales

 Try to get an all cash deal, but you will likely get a percentage of gross sales for some number of years. That way the buyer has some assurance the clients will come to them, rather than shopping for a new preparer.