I recently bought a McKay UZI receiver and a bunch of parts from a fella, 2K miles away. Found it on Armslist, he didn't offer shipping in the ad, but I asked, and he was amenable. We worked out a deal, and while he could legally ship to my local FFL, I asked my local FFL, just to be sure, and they weren't willing to accept a firearm from a non FFL holder for their own liability reasons. I informed the seller, so he shipped through his local FFL to mine. I offered to look for another FFL on my end, willing to accept a firearm direct from him, but he decided to just ship through his local FFL to save me the hassle.
Finances were handled via Paypal, while they're not firearm friendly, we kept things vague, I wrote "machine parts," in the notes about what I was buying, though I think you can leave everything blank. Additionally, you can pay via a credit card instead of your bank account through Payapal, so you have another layer of protection if something goes sideways.
Everything worked out, but there was a decent amount of communication, at first via email, and eventually text between the seller and myself, so I was 95% sure he was legit, before committing funds to the transaction.