GP practices have always been private business (pre- and post- NHS formation). They are essentially private businesses that hold NHS contracts for work. Unusually, as a business, they typically ONLY have an NHS contract as their source of income (CF a business with only one customer). This creates a slightly unusual behaviour as the buyer and the consumer are no longer the same, and market-type behaviours no longer function efficiently. Buying and selling of businesses is essentially no different whether they hold NHS contracts or not (though with some additional regulatory steps if they wish to continue providing NHS commissioned services).