There's a reason why lots of people have this idea, but nobody ever does it...
Ribble do just that, buy frames in bulk lots from abroad and sell them on with the punters choice of groupset. I suspect they also buy their groupsets from abroad rather than go through Madison.
They get the Groupsets under OEM the only difference is no instructions and in plastic bags. They are supposed to be only used on new bike builds. Bit like the old OEM windows that you get with prebuilt PC'sRibble do just that, buy frames in bulk lots from abroad and sell them on with the punters choice of groupset. I suspect they also buy their groupsets from abroad rather than go through Madison.
Selling the bikes as used "due to test ride" would probably be the way forward
It's a used product. No different from when car garages sell stock cheap as used because it's done 15 miles. Is there much difference between buying a bike, taking it out for a ride, realising it's the wrong size and selling it?
Not making any money from it so income tax isn't relevant
not doing it for profit
Can I ask why do you want to do this?