You'd have no problem selling your cartridges as they are not controlled by the new rules in Great Britain.
I could buy your rifle (as the new rules have not been replicated on the Isle of Man) but you (strictly speaking) could not sell it to me. The nastiest part of the new rules is that even a Section 5 dealer can't export them.....
....although no export licence is needed on the Isle of Man as it's treated as a common territory with the UK for Customs & Excise purposes
Adrian