I also had a very bad experience with him at NYCC last year as well. He's someone whose work I loved as a kid and had never had the chance to meet him before so I was excited to get a bunch of books signed and maybe buy some art. As others mentioned there was clearly some issues going on that wouldn't be kind to speculate about.
The first day I saw him he mentioned how he had difficulties getting in on time from some sort of train delay and then his airbnb had cancelled on him without telling him so he had to scramble for a room the night before so I chalked up the subpar experience meeting him to him having a very long prior night and a bad day, happens to us all.
On day two he had a ton of art pages all labeled with prices on the top corner but Spencer's people said they weren't collecting the money, only Dan was, and he kept going missing so I waited over an hour for him to come back. Most of the pages were way above what I wanted to pay or characters I don't care about but he did have one old X-Men page he inked for $275, not expensive by any means but I try and stay under $300 so it was already at my ceiling that I feel comfortable spending on a page. He then told me it was $1,000 and I obviously balked. He then said $700 and I said no, I can't do more than $300. He then tried haggling at $100 to $50 increments until he came down to $300. As a sales guy I don't mind negotiating but the whole experience left me grossed out. If you have prices marked on everything you should be honoring them, unless it's some crazy and obvious mix up.
I watched Dan do this to a whole bunch of other collectors as well who just paid whatever price he wanted which seemed crazy to me.
To top it off I'm a big signed book guy but only try and do a five or so at a time. He was my last stop before leaving the con after seeing him several other times over multiple days. I had already given him a ton of money for signatures, ($5 each). So I had two books left and he signed them, I only had $20s left and he said he didn't have change so I said to him, go enjoy a beer on me and keep the other $10. He thanked me and put the money into his super full wallet with a ton of $5 and $10 bills. Obviously he knew how much money he had and I left annoyed thinking I guess you need that $10 more than me.
Everything about it rubbed me the wrong way and on the rare occasion these things happen, I always hold out hope it's just someone going through something or they're having a bad day. He only got me for $10 so I can't even imagine how pissed you are.