I'm not sure why it's different when the situation is reversed. A couple years ago, there was a board member that picked up a T206 Brown OM back (from a PWCC auction) for about $51.
Here is the original thread (save yourself a lot of reading and pick up from post #30).
http://www.net54baseball.com/showthread.php?t=173902
Post #85 is where the board member comes forward as the buyer.
If you're going to inform an uneducated seller about the value of a card, does it matter if that seller is PWCC (the Brown OM back), or a relative of a deceased collector? I really see no difference. And I'm not saying there is a right or wrong answer, but I find it interesting how people justify things based on the roles of who the buyer/seller is.