A restraint of trade is a promise you make as part of the sale limiting what you can do afterwards, usually around competing with the business you just sold or poaching its people.
Technically, it is a contractual provision that limits a person’s ability to engage in commercial activity, prima facie void at common law as contrary to public policy, but enforceable to the extent it goes no further than necessary to protect a legitimate interest of the party it benefits.
The buyer wants the restraint as broad and long as possible, protecting the goodwill they just paid for. The seller wants it narrow and short, limited to what actually threatens that goodwill, because an overbroad restraint can shut down your next opportunity long after this deal is done.
Restraints given on the sale of a business get a more permissive enforcement standard than restraints in employment contracts, because the seller has been paid for accepting the limitation. That standard is shifting: the ACCC’s newer merger control powers can declare a sale-of-business non-compete unlawful, and federal reforms targeting employment restraints, though currently excluding business sales, signal the direction the law is moving.
What you as a seller should push back on: a restraint defined by reference to the buyer’s future business rather than the business you are actually selling, geographic scope wider than where the business genuinely operates, and any restraint that would functionally prevent you from earning a living in your own field.