You'll do a paper form 4, only FFLs can initiate an eForm 4.
You are the Transferor, the dealer is the Transferee. You can fill in your part and they can fill in theirs. Either you or the end buyer is on the hook for the $200 tax here, and yes you keep the can until it comes back approved.
I'm assuming "your SOT" is in your state, so from there they will transfer it to a dealer in the end buyer's state using a form 3, this is tax free and should clear fairly quickly. Then your buyer will get it from his dealer with another form 4, that one will be the same process as if they were buying a new can from that dealer, just that you got the payment for it obviously. At the end of the day, it's two $200 taxes paid, two from 4s and a form 3. Make sure all parties are lined up and aware of the overall plan as well, since it will involve two third parties (the dealers).