Hi Jeff: I've used Amazon a great deal and Archiv occasionally. Sometimes they have a good bargain, but I use them for cds out-of-print that don't seem to be available at the time I really want them. For years I have bought and sold on Ebay and have a great many in my collection over the years from them and have sold many items on that site including cds. I would also recommend another really great site Berkshire Outlet http://www.berkshirerecordoutlet.com
This online store is fantastic with great prices and only sells new items (some cases may have a small hole on the back of the case or a cut on the side of the case, but that isn't the norm) and their offerings are staggering and include some SACD items, DVDs of concerts and operas and occasionally rare items in great demand by classical collectors. I have made a great many purchases from Berkshire over the years. Take a look at this site and your classical world will expand immensely. Good hunting...Lee