We visit a few auction places in the North West/North Lancs/North Yorkshire and I'd reckon that 50-60% of them carry this kind of stuff.
Seen the same bust of Hitler twice recently
As someone of Polish/Jewish heritage who has relatives (that I never knew/met etc.) who dies in various camps, I find this stuff repellent but understand the morbid interest it garners.
I can only suggest that you contact the relevant Council's trading standards office if you feel that it needs addressing. If you feel that the items are not original and therefore have no legitimate 'historical interest' then perhaps they may have a stance on the sale of such items. Same for 'real' memoribilia.
If they do not have a stance, perhaps ask why not, especially in light of recent events in Ukraine. Will the auction house in question deem it acceptable to sell Russian Z flags any time soon?