They are claiming that the sewer line was an issue that my company and the listing agent had knowledge of and did not disclose the information. 

Every fee should be fully disclosed!

But if you disclose the name of the lender we can give you some thoughts as to whether or not they are legit.

Forgive me if I double-post here & in my signature, but I need to be sure that I disclose properly. 

I'm going to disclose and move about my business.

I'm not a lawyer, no legal advice, but it seems to me that if it is not in the contract or wasn't disclosed to you as a closing cost, you should not be held responsible. 

Disclose that an inspection has not been done and save that money for another deal.

Clearly if I have marketed on my own ( as I am  always looking for properties to add to my retirement portfolio)and I  clearly disclose to this client that , yes , I do have an license , but in this situation I am not acting as a real estate agent capacity but as a private investor. 

Do I disclose the seller taking a second mortgage?

The fee they would have made on the kick back would have doubled their income on the project.Their kickback fee was never disclosed.