Bike jumbles are an odd place. I am a bargain hunter and will flip if theres a profit -
only because at one point my mortgage depended on it. Now I'm back to hobby status its amusing to watch the behaviour of certain others plus slightly embarrassing to realize that I too was behaving in the same way.
Back at the jumbles, watching a certain luminary overload his car then realise that he'd too much and try to sell the overflow
back to the bemused stall holders summed it up - I didnt want to be that guy anymore.
The people that still do this are in a grey moral area - if you think its too expensive, dont buy it - thats the only advice I can give.
I still make a 'profit' (not counting hours cleaning and rebuilding) but its finding a bike for £20 - cleaning and making it rideable that I still enjoy. Punting it out for maybe £50 - £60, sometimes including postage means someone gets a bike they can enjoy or pass on for a profit themeselves.
I cant condone what I see in the others - small parts at insane prices, immaculate bikes stripped with forks and frames sold separately its not what cycling is all about - one of the most environmentally sound hobbies/ passtimes/ whatever on the planet monetised to the extreme by the amoral few.
I remember selling a huge pile of stuff a few years ago (stuff I should have kept come that horrible jobless time!) and the lucky recipiant put quite a bit on ebay - a few were indignant on my behalf but I'd sold it all for that very purpose - someone in a vulnerable position was able to make a few quid extra due to the generosity of someone else.
I've benefited from this too, lots of karma gifts over the years from some very generous people for the very purpose of making good and selling on to keep the wolves at bay. I have inadvertently upset a few too as parts were sold that shouldnt have been
but I have to live with that - I do actually have a conscience!