These 'sellers' appear to be simply people who use the site for the usual social media reasons, and will offer items which they claim are obtained on their visits to premiers etc, often showing images of famous people signing items at these events.
All very believable, but how genuine are they?
There are of course some genuine dealers who use this way to promote their business, and there is nothing wrong with that, although you should always check the sellers membership with any of the associations etc. Many are not registered as a business, and can close their account any time they like. These 'private' sellers are a very different kettle of fish though, as based on what i have seen they are often offering fakes at a tempting price with a convincing story, so if you find out the truth later you will have zero chance of getting any kind of refund.
And never ever pay anyone on facebook via bank transfer, its a sure fore way of losing your money!
PayPal should be your first choice as at least that way you have some means of getting a refund if it all goes wrong.
So my advice has to be, be very wary of any seller of signed items on Facebook.