Parts - buy/sell to help fellow members or make $?

I don't see anything wrong with buying a part for cheap, and then selling it for a little more. If said part sold here for $20, but was worth a lot more, and somebody bought it and re-sold for $75, I don't see anything wrong with that, if the part was worth $75. I usually lose or break even because when I buy parts, it's with the intention of using them, but sometimes, my intentions change. I'm sure there have been people who made a profit off stuff I sold them. Good for them.