We are dealing with collectible items thus if someone wants to buy an item he has to do his homework.
Concerning signatures, as in all Art field, no certificate= no signature. It means that no added value can be brought to the item if not supported by a « valid » certificate.
Newbies can PM any member to ask for advice. It has been done numerous time in the past and will surely be done in the future. It is the best way to check the value of an item.
It is very easy to check the profile of the seller.
I have not seen any serious complaint about NMB sales in 20 years.
In fact, NMB is probably the safest and best place to strike bargain.
And as you said it Adam : « I doubt many would buy an item with signature and not check it's authenticity. »