Armlist is pretty local and will probably mean face-to-face or phone contacts, I would use Gunbroker or similar for a larger marketplace and distant sales, just ship it and forget it.
For gun parts and holsters, you could use Ebay and PayPal. I've bought a slew of holsters that way. Be sure to identify the exact pistol the holster fits, including make, model, and barrel length, if you just advertise it as 'Fits S&W .22LR revolver', or some such generic language, folks will pass on it.
Same with gun parts.