human nature:
a known bike buyer/seller (flipper-1) has a bike that can be realistically be sold for 2000. let's say he offers it to flipper-2 for 1000.
if the flipper-2 knows that flipper-1 bought the bike for 10, he may be reluctant to buy it or at least haggle, knowing that flipper-1 is making 990 on for doing next to nothing. OTOH, if flipper-2 knew for a fact that flipper-1 had paid 1200 for the bike, he would snap it up in a heartbeat.
just a long-winded definition of greed really. a superficially unattractive evolutionary trait shared by all mammals. rarely involving offspring.