Never really got the whole 'bug someone about buying something, then fuck off and not buy it' deal. If I see something that interests me, i'll sit there for a little while asking myself "Do I really want this? Is this what I'm looking for? Is this what I need currently for my build/rig/pile in the corner?" Then I'll send the person a PM saying I'll definitely buy it...and then buy it!
Could understand if you settled on a deal, and then suddenly Oh no! I've been robbed have no cash. But stuff like "Yea, I know I said I'd buy it, but now I don't want to" is kinda bullshit.