Both the No4 rear sight & the SMLE stock set seller are forummers on here. I don't know about the SMLE stock set, but the No4 rear sight seller had quite a large quantity off me, so I can assure you they were truly 'spares', at least by the time I acquired them twenty five years ago. No healthy No4 rifles suffered in those particular deals! And, I managed to get some very useful SMLE spares out of it.
But I take the point about silly prices in general. Sadly, whilst some people are prepared to pay ridiculous prices for spare parts there will remain an incentive for others to break up good rifles. It's a minor tragedy.......
& maybe not so minor........