They don't have to testify...I saw your plaque sale and the prices listed. If you got that for them then good on you, if you didn't then maybe deals were made, either way they were not "cheap". Funnily enough I based the "prices" on your sale as there were (as you say) comparable pieces although (also) funnily enough, no Clermont plaques.
I also know that one of the ISC plaques was originally bought for about £100 before being sold on (I think that 2 of the Hyde Park ones were a similar price) so prices can change depending on many things, can they not?
Glad to see I got exactly what I expected but hey.......