I look at neither because:
1) I don't need any more cars and there's always eye candy on BaT

2) They're not really cars for me as most of mine are drivers. I don't really believe in buying mint no mileage cars, although I know BaT more well "used" cars either. But then I don't see the point in paying the higher fees.
I would say all my M's are on the rarer side, and I've found 2 of them on Autotrader (both dealerships) and one was local Facebook ad/car group.
- E92 M3 Sparkling Graphite/Bamboo Beige with a CF roof and 6MT
- E36 M3 Techno Violet/Dove 5MT
- Z4M Roadster Alpine White/Red, no nav
There are still plenty of deals to be had, granted only 1 of them was an enthusiast owned car which might be important to some people.
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Current:
16 F82 M4 GTS, Black Sapphire/Black, DCT
08 E92 M3, Sparkling Graphite/Bamboo Beige, 6MT
07 E85 Z4M Roadster, Alpine White/Red, 6MT
99 E36 M3, Techno Violet/Dove Grey, 6MT