Yes, but these sales should be otherwise legal with records of the transaction now kept by Facebook.
Most states require you to show firearms purchaser IDs, called FID in my home state as well as a drivers license.
To acquire a FID, you need to go through a background check. This is how you are meant to know if the person is "safe to sell to".
If people in state X have a problem with that, then they should get together and find a solution to it.
Until then I don't think I can justify to myself censoring an otherwise completely legal discussion, especially when such a discussion can be used as a record of a sale that would otherwise not be present.
That's just my two cents.