
I think everyone makes mistakes, from people with much experience to people with little. What they do when they are called on those mistakes is what matters. The technology age gives the consumer more resources than ever before to do homework on an item before they purchase it. Forums like this one allow for niche collectors to gain greater insight into a product they may wish to purchase. If a mistake has been made and the buyer calls the seller on it, the seller should refund the purchase to make things right. Since we are talking about MEARS in this thread, I'll use them as an example - they have said they will refund your purchase. If you look at most auction houses, all sales are final and you are pretty much SOL if you later find the purchase you made was not authentic. Sellers should back their products and when mistakes are made, should make changes.
Like you have also stated, overselling a product is nothing new. I know one dealer my dad has purchased some gloves from and had to send them back because they had a difference of opinion on condition. Condition is subjective but I do agree that facts about an item are not. However, sharing information in this technology age is easy and letting a seller know something you know that they may not, regardless of knowledge base, is much more productive to the collecting community as a whole.