For a while now Ive been running into ffls that wont accept a shipment from a non-ffl. They are requiring the gun to be shipped ffl to Ffl, which (sometimes?) requires a transfer on the sellers end. The reason is if the transfer is denied on the buyers end, their ffl has nowhere to “cya-legally” ship the gun back to, so they’re stuck with this gun that isnt theirs which will potentially cost them time and money to get rid of. This is NOT the law, but it’s the policy of many fffls Ive run into. As a seller just specify cost “direct from me to your ffl”, and you should have a copy of the ffl you are shipping to so if there’s an issue you’ll know before shipping. It’ll just cost more to ship thru an ffl.
All of this info is available online, I would encourage you to research it yourself direct from the source and not rely on what you read from random dudes on the internet.