No, you do not have to use a lawyer when you sell your business, but I would strongly advise you do. There is a lot of potential risk with the sale (and purchase) of a business, and the lawyer’s role is to help you recognise and mitigate that risk.
Selling a business sits on a continuum between selling something like a car – where lawyers are generally not involved – and a house – where almost everyone would involve a lawyer. Sometimes, you are only really selling some assets when you sell your business. This is akin to selling a car. It is a simple exchange and ownership transfer, and no lawyer is needed. Most often, there are many, many other factors and potential risks and issues, and you would be foolish to try and do it all yourself.
Yes, you should use a lawyer.