Make yourself know to the buyer. They might contract you for future fixes. I helped work on a startup a few years ago that changed ownership. Like you're sayin

Make yourself know to the buyer. They might contract you for future fixes. I helped work on a startup a few years ago that changed ownership. Like you're saying, documentation wasn't great. After hiring devs to work on it that couldn't make any progress, they hired me to make some emergency changes. I also offered to improve their documentation and setup a proper release process so that other developers could work on it.