If someone says "make an offer", and has no clue as to what a fair offer is, why would the buyer be obligated to disclose anything? There is no deception. The seller should do their homework. As long as you have not misled the seller , you are not obligated to disclose your knowledge.

Now that being said, I once walked into a consignment shop and purchased a very valuable collection at a fraction of its true value. I paid what they asked for it. I didn't do a darn thing wrong, but it sure felt like it. How do I know that? Cuz I couldn't wait to get out of the shop with the binder. Believe me, it was worth at least 25X more. After the initial giddiness wore off, I felt guilty as heck. I ultimately called the owner (an older retired gentlemen)and fessed up. He floored me with his answer---he told me "Son, that is the way the cookie crumbles. I didn't care much for them anymore, I went thru a lot of money in my gambling days. I am glad that you like them so much though!" And that was that.

Everyone has their reasons. Good luck with your purchase, love to see what ya got!