hidden value??

Can an agent offer an xxx amount to a seller to buy his property without disclosing all the value of the property? I had a fellow agent ask me this question. She thought that it was not ethical to personally buy a home from someone and not let them know that there is hidden value in their property. I understand when you represent a buyer that you must point out all pertinant information. I understand that if you represent a seller that you let them know what their home is worth in order to get fair return for it. But isn't ethical to offer any amount if you are buying the property for yourself??