I guess what irked me (not necessarily today but recently) is when a forum member negotiates a deal for something but then doesn't respond to PMs to complete the deal. I had a couple of interactions with this person before and should have known they would flake out, but you try to give people the benefit of the doubt. Needless to say, this person will be on my ever growing list of people to NOT do business with, regardless of what is involved, because of their inability to conduct themselves in an adult, business like manner.
The shrinking number of people in this hobby that conduct business in a decent way makes it harder for me to participate in trades. I have done significantly less trading and dealing with people here this year and am at the point where I am better off selling or trading at shows instead of online.