I found this ad in FF #52 (July 1966):
Looking backwards from there, I don't see any comic dealers advertising in FF #48-51, so I'm going to guess that was the beginning. Note the $50 offered; that was probably for issues like Action #1 and Marvel Mystery #1 and Detective #27.
In Blackhawk #202 (November 1964), editor George Kashdan offered to set up a trading corner on the letters page:
And in Blackhawk #206, we saw the beginnings of this new section:
This became a very popular feature in Blackhawk, showing the demand for back issues that dealers soon rushed to fill.