Here is, as she says, an interesting "moral quandary" from jumpingjessflash. She was making her way through Maraudon (one of my favorite instances) with a few guildies, and the BoE epic Icemail Jerkin dropped. At first, she thought it was an upgrade for her Hunter,
so rolled Need and took it she asked for Need, but then eventually won it with Greed (see note below). But on second thought, she didn't like only +5 Agility-- turns out it wasn't the Jerkin for her.
So she's left with a choice: wear it anyway, or auction it and risk suffering the wrath of her guildies.
I've been in (almost*) the same position, but (especially with guildies), if I'm not sure whether I'll use something or not, I'll usually let everyone else in the group know that it's iffy for me-- usually they'll let me take it anyway, to keep as a sidegrade or a piece of a different set. If I want to be really nice, I'll sometimes let them reroll on it if I decide I don't want it, and then send it away to the winner. Another option is to sell it, and then split the gold.
Of course, you could also go the other way and send it to an unguilded alt to sell it anonymously. That way, you get the cash and no one is the wiser. Like I said, usually I don't care, but I'll admit that sometimes I'm a stickler-- if someone happens to roll Need on a BoE that I think is a questionable call, I have in the past asked them to put it on so I can see them wearing it via /inspect (I haven't ever done that to guildies, but in a PUG everything is fair game as far as I'm concerned).
Still, I don't have a problem with selling a piece picked up accidentally, if someone thought it was a good piece and then had second thoughts. If you did that and then put it on the AH, no one would accuse you of being a ninja-- would they?
*Update: As a few people have pointed out (including the livejournal poster), it seems like I did misunderstand her original post. She wasn't ninja-ing at all-- she asked for Need, but eventually won it rolling greed. Her concern was that someone may have passed because she asked for Need, not because she rolled Need and then decided she didn't need it. Because she rolled greed and won it, the piece was hers.
Anyway, I still think the issue of second guessing something that you roll Need on is an interesting one. That's not what the original post was talking about, but it's what I am talking about in this story, and I think it's worth a discussion.