A bolt is a component part. Restricted component parts are listed in Law. You can not possess a bolt without lawful authority. (see SD's passim)
To be in lawfull possession, component parts must be named on your FAC as aquired.
The guy can not take a deposit on a firearm unless he has had sight of your legal authority possess.IE a properly conditioned FAC. If he does, he breaks the Law. He can, however, give you a receipt for your cash
A trigger assembly is not a listed component part and therefore does not require authority to possess or an RFD transfer
In this particular circumstance, authority to possess would be via the firearm already being entered on his FAC. If the FAC was simply conditioned for aquiring a firearm, then he would be in illegal possession.
The chap who imported the component parts from the USA did so legally, untill he ordered the frame and the barrel. Both are listed restricted items, and he was then arrested. The seller in the USA, Gunparts, informed UK authorities once they realised what was happening. The UK authorities requested that Gunparts continued with the transaction. This is now irrelevant as the USA Patriot Laws subsequently prevent the export of listed component parts.
If someone holds a listed obsolete calibre firearm, then its component parts are also classified as obsolete. If, as suggested, someone was minded to put that bolt in a licensed firearm, then it form part of that firearm and would have to be entered on to an FAC which was conditioned appropriately.
Remember, guidance can be "interpreted" but Law can not.