When you contract to sell your newsagency, or any business for that matter, you usually contract to sell the business as it has been trading and you contract to sell the assets of the business. Your business data is part of that trading, your business data is an asset of the business.
Removing data prior to the completion of a sale transaction could open you to a legal challenge. At the very least is would raise questions.
There have been court cases between purchaser and vendor over data on which the purchase valuation was based. These are complex cases from which neither side wins. In one situation the case went of for years, costing well into six figures in legal fees.
If I am ever asked, my advice to newsagents selling is to NOT delete or tamper with data on which your business valuation is based.