I've accumulated a fair number of cool comics over the years, including a few "key" issues. But my first big score came around 1969, during the summer between eighth grade and high school.
As a (mostly) DC fan, there were a number of issues that I desperately wanted to buy. Action #1 and Detective #27 were prominently on that list, of course, but I'm talking about comics that I thought I had a reasonable chance of finding in a garage sale some weekend. And among those, two of the most important were Flash #123 and Secret Origins #1.
Flash #123 was the issue where it is revealed that all of DC's Golden Age heroes--the Jay Garrick Flash, the Alan Scott Green Lantern, etc.--actually lived on a separate Earth from their Silver Age counterparts. Barry Allen gets to meet Jay Garrick in that issue. I had read most of the story in an 80-Page Giant, but a few pages were missing at the front of that book so I was anxious to read the rest.
Secret Origins #1 was DC's third 80-Page Giant, and contained the origin stories of most of the members of the Justice League of America. They didn't undersell on the advertising for that book:
So anyway, my local newspaper had an annual event called "Kids' Classifieds" where kids could advertise for free anything they wanted to sell. When that paper came out, I was on the phone dialing up all the kids advertising comics for sale. About the third one down, I hit paydirt. This kid had Flash #123, Secret Origins #1 and a whole slew of other DC silver.
One problem: The kid lived in Hillsdale, which was probably 8-9 miles as the crow flies from my parent's house, and it was not a straight shot on any road, so by bike it was probably going to be 12 miles or so. And the guy was not willing to hold the comics for me; he said "first come, first served." So I told my mom I was going up to Ramsey (which was the next town up) and pedaled furiously to his house. So I get there and we pretty quickly made a deal; IIRC it was something like $15 for the lot. In addition to the two biggies, he had Green Lantern #2 and #3 (coverless), the first two appearances of the evil universe of Qward. There were also quite a few issues that I already had in my collection.
But the best part of the story is what happened next. Another collector showed up just as I was leaving, and bought the duplicates off me for the same $15 I had paid the original owner. So I walked out with a bunch of cool comics for nothing other than the treadwear on my bicycle tires. :)