Agree with this 100%. I used to lead these calls as a mgmt. consultant for mid-market PE involved transactions and this was almost always our angle. Very important to note though is you'll still get the selection of best /happiest customers (as brought up in other comments) and secondly, your question set should be aligned with the positioning you're going in with to avoid setting off any trip wires (some customers are very savvy and may sniff out a potential transaction happening). Good luck!

Mike Adhikara is 100% correct. Anyone that doesn't understand this has never been a seller nor sold a business. If the buyer goes behind the sellers back and calls any customers, that is a complete breach of trust, and the seller should back out of the deal if not sue the borrower. This is how legacy businesses get destroyed. You are dealing with humans which are wildly unpredictable. If you just happen to call one that has an axe to grind, that one customer can call every competitor in the seller's area and tell them that he is selling because he is going broke or some other stupid reason. And before you think I'm exaggerating, yes people do things for malice for whatever reason and have no problem causing others a complete nightmare. Not saying DD isn't important, but there is a wealth of knowledge on the Web where you can find every stat known to man. If the seller is pushing back, you need to respect that. Learning how to improve the business model or customer satisfaction can be done post-closing and is also a good time to "touch the customer" with a new face. This may also make a case for a solid QOE.