About twenty years ago my father and I went to a older man's home who was a huge dealer that had lots of rare gold coins and key dates etc. Well over $100,000 in inventory he spread out on his kitchen table. It was mindblowing! You just never know what folks are carrying.
Nowadays I don't see how dealers can sleep at night because you can't buy insurance to cover all that, it is too expensive.
Also home owners insurance normally does not cover much by way of coins or bullion or even firearms if they are stolen. If they do the limit is so small it is about nothing.
I did run across a company one time that did cover collections.
http://www.collectinsure.com/Collectors may purchase coverage for Gold & Platinum coins up to $20,000