I just revised my educational page on how to buy ancient coins: http://augustuscoins.com/ed/numis/buying.html If you are a beginner or a collector who only buys from only one or two sources, you might find that page helpful. Also, it links to this page with links to sites of dealers and auction firms: http://augustuscoins.com/ed/dealers.html It links to a page about eBay as a (not highly recommended) source and how regular internet ancient-coin auctions work. http://augustuscoins.com/ed/numis/auction.htm I love our hobby and want collectors to know everything I know about the best ways to find coins to buy.
