I don't get the "just going to ebay" complaint. What do they care? It's not like it's taking money out of their pocket.
It can, though. That's the thing.
I mean, ok, for a dude like Chris Pratt or someone who's actively getting paid millions of dollars to do this or that show or film or whathaveyou, it's not about the money. But for the folks who work the con circuit, it is. They get paid to sign stuff. So every time they sign something and don't get paid...
These guys, their job, their income, is at least in some significant sense about selling widgets. Those widgets take the form of an autograph. I mean, realistically, do you think someone like Lou Ferigno is getting a lot of acting gigs or even endorsement deals anymore? I think the last thing I saw him on was an infomercial some ten years ago. Occasionally they'll get a cameo on some TV show making a pop-culture reference, but otherwise, the cons are where they're bringing in money. So, when someone says "Hey man, can you sign my poster?" it's like saying "Hey man, gimme a widget. No. For free. Just gimme one."
If they're working at a con, and they've partnered up with some distributor to only sign that distributor's stuff, then they're getting a cut from that or have been paid to do so. In a way, that's worse than saying "gimme something for free." It's like saying "Take something from your employer and give it to me for free."
Now, none of this necessitates them being rude in their denial of such a request, but I can also understand why they might get a little irritated after the umpteenth request.