Facebook has a policy banning private individuals from selling guns to each other on their platform - https://www.facebook.com/help/179037502478035 and "Armslist" appears to be a platform for doing just that: "Gun Classifieds, Guns for Sale, No Fees, 45000 guns for sale. The largest free gun classifieds on the web. Buy guns, sell guns, trade guns."
One could argue that the transaction isn't happening "on Facebook" since it's going through Armslist, but I don't find that convincing. If take payment via Venmo does that mean it's "not on Facebook" if everything else about the sale was?
Anyway, I fault Facebook for plenty and generally avoid it now, but I don't fault them for this policy. Private sales can get into sketchy territory really quick and I wouldn't want to mess with that on a public platform I created unless I was willing to invest in the nuance there which has basically no value proposition for Facebook.