When I was in for the week's new comics at an LCS, back in the 90s, a youngish, teenage guy pulled out a SA Superman Annual 1 and asked me if I had any idea about what it was worth. It was the grungiest, most fragile-looking comic I'd ever seen, covered with pen ink, and on the point of disintegration. I said that I'd recently seen a reading copy of Flash Annual 1 at a comic mart, part-detached cover, complete and with generally very nice presentation going for £10 / $15 and so, thinking I was being diplomatic and generous, I suggested about the same for his book.
His face dropped and, almost crying, he told me that he'd just paid £100 / $150 for it at the comic shop across the road. All I could say was that the owner was fairly well-renowned for this, and sorry.
I enjoy analysing such situations and being straightforward, but sometimes I can end up regretting it.