In many businesses, the proprietary information is one of the most valuable assets; this includes marketing systems, materials, and data. And, once the "barn door" is open, it can be impossible to get all the critters back in their stalls. So, it's a fine line.
The buyer needs to be shown enough during due diligence so they don't cancel and get their deposit returned; but, not so much that they could use the information to the seller's detriment if the sale doesn't go through.
We have used representations about marketing material that, if not realized at closing, will generate concessions to the buyer -- from a sale price reduction all the way to contract cancellation and damages. For example, the seller represents that there is an active database of current or recent unique customer records with 200 having done transactions in the last month, a total of 700 i the last three months, and a total of 2,000 in the last 12 months. At closing, if it is discovered that this numbers are off, the buyer would be entitled to compensation, or, if significantly deficient or non-existent, the buyer could terminate the contract and get liquidated damages to cover attorney and escrow costs, etc.