In most cases, because the number of fakes out there, when people post pictures, it is of a fake. Usually no details of the telltales are given in the public forum. If the questioner wants to know why something was recognized as a fake then they can PM the person doing the assessment. In that case the buyer avoids buying the fake and there is no information given to the faker as to what tipped off the expert(s).
In most cases that's as far as it goes. In most cases someone just wants a quick thumbs up or thumbs down before buying something or making a bid, and most people accept the assessment.
Sometimes, unfortunately, someone has already bought something and they are showing off their new "find". When told they have unknowingly bought a fake, the most common reaction is denial, which can sometimes go to extremes bnz 44 single rune RC - Gunboard's Forums
MINT G98/40 jhv 43 RIFLE - Wehrmacht-Awards.com Militaria Forums
Other times, a minority unfortunately, the item is legit, and again people are simply told that without going into details. Usually such instances are scattered, so it isn't a goldmine of information for a faker. They would have to scan the forums constantly, diligently collecting a database of photos of legit items to learn from. I'm not sure too many bother as long as their wares are selling.
But, bernie-attorney was proposing making it really easy for the fakers by creating a concentrated exhibit of legit markings, cartouches, etc. that they could study should they wish to. Make it that easy for them and they will take advantage of it.
An alternative to posting questions in a public forum is to do it all privately through private messages and email. If you spend enough time in the various gun collector forums you learn who is good at spotting fakes. For example, for K98s I run things past some of the guys on the gunboards.com K98forum using email and private messages.
For newer collectors I think they just have to be constantly reminded of how commonplace fakes are, and that they should run things by more experienced collectors in a forum or through PMs or email before spending a lot of money.
Here is an example CMP Discussion Forum - possible original national postal meter?
People explained their concerns about the carbine, and I sent the original poster an email with a link to a slideshow of the fake cartouche on the carbine he posted along with some real cartouches. He didn't buy it, but someone else on the CMP forum apparently didn't care about authenticity and paid a lot of money just because he liked the cosmetic appearance of the carbine. As long as he knew what he was getting (and isn't in denial) that's his perogative.