We are in the process of selling our house and our buyer's inspector says that there are structural issues in the basement. Therefore she is hiring a structural engineer to come out and look.

When we bought the house 5 years ago our inspector was unfamiliar with foundations this old (150 years) so we hired a basement inspector. The basement inspector was in his 70's and really knew his stuff. He said our basement was great and that it wasn't going anywhere. He also explained to us how the rocks are designed to take on some water and that they had to breathe and are very different than a modern day concrete basement. Our basement even has a trough and outlet to the outside that can hold water if necessary. I don't believe it has ever had water as I've never seen evidence of it but it is obviously designed to allow for that. Additionally there are no cracks in any of our walls upstairs or issues with the floors. There is some spalling (sp?) from the walls but it doesn't seem excessive to me and I thought that was typical and expected from a basement this old. The basement inspector we had said that we should all be so lucky to have such a strong foundation.

My question for you smart folks is:
1) What would be evidence of a structural issue? (My buyer has not shared the inspection with me.)
2) Are there any websites or good information I can forward to her with regard to a basement like that might help put her at ease?

We are FSBO and our buyer also does not have a REA helping her. I offered to have the same basement inspector we used come out but she declined, instead opting to call the structural engineer recommended by her inspector.