Maybe chosing to serve in Starfleet keeps a person from being greedy, but most of the civilians we see on Star Trek are fairly materialistic.
Not sure I agree with that unless you're talking about non-human civilians. The Ferengi are certainly greedy, but I don't recall seeing humans trying to cheat or scam anyone on a regular basis.
You're right Sran. I was just thinking about all those aliens on Deep Space Nine who were quite greedy. Most of the human civilians in Star Trek are well behaved colonists and such.