Hi, the first thing I would do is check out this great site:
http://www.mlbcollectors.com/MLBcapindex.php
Next, I would be skeptical of whatever a seller tells you, and that includes auction houses. The truth is, there are very few experts when it comes to caps. I would ask for good photos, and look for repairs on the bill. The bills can be tricky — those from the 1950s and 1960s have a stiff liner that cracks, and then the bill loses its shape. I've also seen some pretty sloppy repairs of bills. I've noticed that bills with missing threads often have problems. Whatever you do, don't flex the bill of an old cap you just bought.
Yes, it is tough to find pre-1970 game used or game issued caps. Still, I've found quite a few nice ones on eBay at very fair prices — and a couple were flat-out steals.
I started collecting caps about three years ago when I bought an old collection, and I've been adding to it ever since. Here they are ...
https://imageevent.com/ccmcnutt/vint...0&m=24&w=0&p=0
I picked up this 1954-55 Orioles cap off eBay just a couple weeks ago ...