I imagine bits of sets get broken, so you end up with fishes without dishes and dishes without fishes, lurking in charity shops.
Also, sets of things can become seperated when folk are making donations to charity shops, perhaps, because of their shape or size, they end up packed in different bags or boxes, and the sorter doesn't know they belong together.
I found a Tapio Wirkkala decanter in a charity shop one week, the matching glasses didn't turn up for another couple of weeks - in the same shop. Got the lot for £1.80 - decanter 80p, the 10 shot glasses @10p each.
Now you've let the cat out of the bag Michelle - Christine might well be after your/John's fish!