alot of it is pure luck
Ive had plenty
sold an old rusty ax gt, it had 4 months mot left, had oil leaks, water leaks, electrics were buggered, some fool came down and done a straight swap with a P reg corsa, i took the corsa and sold it 2 months later for £1500
ax was worth about 400 tops, nice profit there
current car that i just bought, theres 2 identical to mine on autotrader up for over 5800, well over a grand more than i paid for mine
at the end of the day, everyone looks differently, just because all of us on here know what somethings worth, doesnt mean everyone else does, and some ppl arent clever enough to do research and find out if they are getting a bargain or not