Are we talking about a raw book or are we talking about a CGC graded book?
What condition is the CGC book ? 9.8's are pretty much indiscernible from one another however the appearance of a 3.5 can range greatly.
With comic books I will say it depends.
However, if I go to a show and see something there that is readily available elsewhere (a current Lego set), I will ask, can you match this price? If not I have no problem ordering it and waiting the two days from Amazon and it saves me the trouble of lugging it around the con.
I will tell you what IS annoying and I have mentioned it here. I find it ridiculous that certain vendors charge a NY or Comic Con tax. If you the vendor have the item readily available on your website or in your eBay store at say $70 shipped with free shipping, I am not paying you $85 at the con. Don't even try to convince me that is your best price because it obviously isn't. I will walk away and order it on my phone and you can ship that item to me when you get back.