Dealers of used Leica gear don't come up easily with Google searches, so I've compiled a list. Andrew Nemeth has
a list, but some links are defunct and it leaves off some great sites.
So here is my list for places to buy a used Leica or other classic cameras (US Sellers):
Rangefinderforum Classifieds - Where most Leica users go to sell their gear. It is well laid out and easy to navigate. Users tend to sell at the most competitive prices
Collectible Cameras - A wide variety of Leicas and probably the 2nd largest overall used site behind keh. It also offers some of the better dealer prices.
Igor's Camera Exchange - Igor has a wide variety of Leicas. I think he has the best prices of any dealer.
Classic Connection - A third major dealer has a wide variety at very good prices.
Keh - The largest used camera store has a good selection of Leicas, but they are noticeably higher priced than the above three dealers
George Ury - Has a moderate selection at good prices (better than keh)
Tamarkin - Has a modest selection. (comparable to keh)
Koh's Camera - Has a smaller selection that tend towards collectible rather than user cameras.
Manfred Schmidt - Wide variety of camera choices, but prices are high (higher than keh)
Adorama - The camera superstore also has a substantial used section at competitive prices (slightly better than keh).
B&H Photo -Adorama's main competitor, it usually has a smaller used section than Adorama, but prices are competitive.
Sherry Krauter - The repair guru (who I heartily recommend), has modest selection at varying prices (some seem to be close to market, others much higher)
The following are small stores with small inventories, but worth checking if needed
Bergen County Camera - Smaller selection, but reasonably priced
Pacific Rim Cameras - Better site for camera info. Some cameras for sale, but at a high premium
Kenmore Camera - Small selection, good prices
Jack's Camera - Small selection, if any
Wing's Camera - Tiny selection
Brooklyn Camera - Tiny selection