How can I make sure this seller on facebook is legit?

I follow a buy and selling page on Facebook. Someone posted a photo of an item im interested in for $400. This was 2 days ago. I messaged them to see if its still for sale. He said it is. He only posted one photo. I was gonna ask him i send me another photo with his name and my name on a piece of paper in it. Would that he okay?

How can I make sure this seller on facebook is legit?
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