First, welcome to the forum.
Second, do you have a link to the listing?
Third, I don't know how much I would trust buying a firearm (parts) from someone who can't answer what should be a simple, but vital, question about a 308AR.
Fourth, you would be better served buying an Aero M5 lower to guarantee that it will work with your upper.
https://www.armorally.com/shop/aero-precision-m5-stripped-lower-receiver/