I don't have any stories that play that well when it comes to deals I passed on over the years, partly because there were so many, and partly because it all boiled down to me being short sighted .
I remember buying a Hulk 181 in NM from Blazing in '98 for $400 that would've gotten a 9.6 if I'd kept it, but no, I sold it for £400. Clever me. Also Bob had several other Hulk 181s in similar shape, but I left 'em .
At the same show Vincent Z (pre - Metro) offered me a NM copy of Green Lantern 1 for OS guide (at the time that would've been $2,200). That book would've graded no worse than 9.2 according to CGC, and I was certain it was unrestored. But I passed on it.
In '96 Conrad Eschenburg had a set of JLA 1-20 in NM (they were absolutely spectacular) but wanted double guide. I imagine if I'd bought the set he would've cut a slight deal. But again I passed. At the same show Harley had some tremendous stuff such as an Avengers 9 in NM- for $130. And that was just a random example. And yes, I should've gone crazy, but I'd only be dealing for about a year and half......so I passed again.
How things have moved on, and how often do I kick myself for not having the balls then to bite the bullet and spend the bucks. Ya live and learn......