Prior to 1960 or so, you could buy an amazing collection of things from the drug store. Somewhere I have a book from the 50s on the hobby of rocket design. It contains some interesting mixes for fueling rockets. And this was not Estes, this was the real McCoy. And if you paid attention in high school chemistry and did a little experimenting, you could make some really big bangs. Pipe bombs were easy. Then (and I don't know all the reasons for sure) with the radical bombings and such plus a general "is this really a smart things to be selling" they gradually withdrew such things. Plus some laws were passed.
Now buying saltpeter, magnesium, mercury, etc ... it a wee bit harder.
Where should we set the limits?
Jerry Pournelle wrote a series of SciFi books a while back about this very thing. US and USSR made a co-existence pack and basically shut down research into anything that anyone thought might be dangerous.
As to my personal experiences, the drug store supply was gone by the time I got to high school chemistry in 1970, but my cousin (10 years older) got to have some fun. The only check was the druggist asked the local teacher if it was ok and the teacher told him to trust my cousin. Personally I think we were in more danger around the holidays than we knew. :)
Here's the Estes mentioned above: