I recently bought a Schlumpf Mountain Drive off someone on another forum. It was "only" £200, which is very cheap for one of these esoteric bits of kit. I know I should have delved into it a bit more, but the seller did specifically say "it would fit almost any bike" without having to go down the chamfering route because it had a torque arm. I duly bought it. Anyway, it turns out to be the Brompton-specific model, which fits Bromptons easily but other bikes with great difficulty - and a very significant outlay.
I'm as much to blame as the seller, who did actually offer to refund my money. So I'm now £10 poorer (P&P to return it), a little wiser, but very disappointed, as my Pompino cannot now have a Sturmey Archer S2C hub/Schlumpf combo that would have eliminated the need for every cable bar the front brake.
I'm now going down the SRAM Spectro S7 hub gear/coaster brake route, which eliminates one cable, so I'm still not sure what I'm grumbling about, but I will anyway!