I do not know of any commercially available FN AR barrels, new. Probably what you are finding are surplus M-16 A2 barrels. They are a crap shoot. I am not sure, but DoD may be upgrading the receivers to M-4/A-4 specs and the barrels may be take offs. A take off from a lightly used rifle may be a good deal.
As car as legality? I do not know of any prohibition to buying/selling/possessing one of these as the barrel is not a full auto part.
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"Robo is right" Fuzzball