Ideally you should be able to see the customer names or atleast the QoE firm should be able to analyse those data and after the due diligence post LOI. You should be able to see the customer list. If not then its better to move on to the next listing.

We typically leave things blinded pre-LOI, but this is typically how we'd handle the situation post-LOI if the owner still wants to wait to disclose. It may make sense to have certain checkpoints in the LOI as others have mentioned to ensure the information is released before you're on the 1 yard line. We've never received pushback when employing the strategy David mentions, but if the seller is not willing to share customer names at some point during the diligence process, then it definitely makes sense to walk. Working with a good seller is almost as important as buying a great business since issues will nearly always come up post-closing that need to be handled, and this is just a small flavor of how big the stakes can get.