I know that when Les Emery at Norvil stamps up a frame plate he insists on seeing the bike and the logbook to be 100% certain it is above board.
I know this because he did one for mine.
I can't think of how any specific offence would be tied to simple sale of a frame plate, unless it didn't relate to the bike for which supplied in which case you could be open to several aid and abets, and possibly conspiracy if you were shown to be dishonest in what you were selling.
While it is great to think that you can get any spares for any bike, I think as a supplier of these parts you could easily get taken for a ride by the less scrupulous, so in many ways it is a bit naive just to bang them out over ebay. More 'caveat vendor' than 'caveat emptor' I suppose.
(I knew that Latin A Level would come in handy one day, even if I did only get an 'E'!)