The places where I like to buy postage stamps online are:
Ebay (stamps.shop.ebay.com) and Delcampe (stamps.delcampe.net). There are many cheap stamps and I received (by mail) almost all stamps that I bought there. I usually pay with Paypal (when using ebay). When using Delcampe store, I also can pay with Skrill (ex-Moneybookers), that is a service to send and receive money online.
I usually prefer buying stamps using Delcampe store, usually they are cheaper and have more stamps available, both new, old, used, unused and I found there some really beautiful, rare and great pieces to my stamp collection.
The picture shows a screenshot from Delcampe marketplace website, a place where you can "buy now" stamps (fixed price) or purchase stamps by auction. You can sort results by price, recent stamps or closing soon (auctions) and also search by category, country, stamp name and full description.