They Don't Do This Anymore...
Browsing the fabulous Digital Comics Museum archive, I came across this on the inside front cover of an early-1941 issue of Whiz. The reproduction comes from microfiche, but it's still readable.
Two things strike one. First, that the comic lists complete home addresses of the would-be traders. Secondly, there are some pretty wild items for trade. Notable are several guns (shotgun, .22 rifle), a hunting knife, and a 16mm projector (its owner wants to trade it for a printing press). It's like the pre-war ebay.
I hate it when old fogies like me carry on about how things used to be different harrumph blah blah. But there's no denying some things have changed. No publisher would dream of printing kids' home addresses these days. Back in the late 60s Marvel did so in all their letter columns, apparently without major problems. I had a couple of letters printed, and never received sleazy advertisements or letters from pervs. On the contrary, I received a copy of Jerry Bails' legendary Guidebook to Comic Fandom, through which I was introduced to the world of fanzines and comics collecting.
Wait a minute! I was sent a gateway drug leading to fan addiction! Guess things weren't so safe even back then.