My friends and I traded comics all the time as kids back in the '70s, and it was fairly simple: if somebody had a book I wanted, I had to pony up the right books to get it...and vice-versa.
Up until a certain point in time, that's all there was to it, and it never had ANYTHING to do with "what the book was worth" monetarily -- I mean, they were all worth the 20 or 25-cents we paid for them, right? Content/story/art/need-it-for-a-set ruled the day.
Of course, that all went to hell the moment we discovered the OSPG, and a few early comic books stores which were selling back issues for more than cover price...