Starting about, oh, 1960 or earlier it was comics, then a decade later into the book trade via the family, scouting away like mad. Golden Age, ten cent comics were already being collected by 1960, primarily Superman I think. Scouted ten thousand sales after that, often in teams. There were want-lists of books, no internet of corset, so bookdealers had to care, and had to dun places like Universities for their wantlists. Often an ex-student would have a published work, or a poetry folio, or whatever, that was signifigant. If you went to a yard sale and found this item, it may well have been bypassed by a hundred people in a row because it looked like nuttin'. They would buy anything, useless science or school books from the 1800s - and leave any fifties stuff that had no famous name on it.
Curiously the internet sent the price of a lot of rare books down, by emptying the basements of the world all at once. I still look, but the odds are against anything really great sneaking through anymore. Only when someone estate comes up is there a chance... and even then they have to be rich enough to not even care about typing book titles into eBay, and sell you the lot, the basement full of boxes, the whole shebang - then you may find a Tarzan in dustjacket. Not! *
SF hardcovers? Way beyond my price range. But paperbacks, well... my collection (now GORN) was... it was .... I'm sorry I can't continue talking about it.... it .... >>>> (tearstains HERE).