Well we want profit, so "economically" you buy low and sell high for profit.
It is not a scam because customer did agree with the deal.
the moment you bought a pokemon (low price), that pokemon will be 100% yours. the previous owner do not have privilege anymore after the deal conducted, so reselling it for more profit also an option, or drag the pokemon to recycle bin.
for an advice, research market price before you sell something, to prevent low ballers.