ZOS at one time seemed to have possibilities but there only seem to be a few dealers using it. If you collect foreign Delcampe is an option but there are mostly foreign sellers, many in Europe. So shipping tends to be high, sellers charge fees for using paypal (who then charges you for currency conversion), and overall its a bit more of the "Wild West" than eBay
. Be extremely cautious and only buy from sellers who accept paypal.
The stampworld catalog website has sellers as well but frankly I've never bought there. I wouldnt touch a thing on that site unless the seller accepts paypal.
One last idea is bidorbuy.co.za. Its based in South Africa so not a lot of USA stamps to be found.