In terms of hammered coins, I think Timothy Cook has put together one of the best lists I have seen, many of which are BTNA members - not to mention that he has hyperlinked them all such that you can just go down the list at your leisure and click to each shop/auction house. As you can imagine, there are also many other pre-decimal coins and even decimal coins at many of the websites. Most of the links are for fixed-price shops, but there are also the major auction houses sprinkled in throughout the list. Specifically, (like @TomGoodheart mentioned) he has called out helpful websites like NumisBids that act as platforms for viewing lots from multiple auction houses at once (not to mention you can also sometimes submit bids through NumisBids).
https://www.englishhammered.com/links.html