I may be a naive fool, but I think they really just didn't quite get it right on this (and the s/h instruments market, discussed in other threads); after all, the agreement document is loads of pages of gobbledegook.
I don't think the agreement was intended to put minor musicians out of business, nor stifle the 2nd-hand market.
They probably spent so long wrangling over fishing rights and NI that they didn't go down into this aspect of the minutiae.
Writing to your MP should help bring the matter to their attention and maybe move towards getting things put right.