You might want to read
this page, to get a general idea of what the website's about. Individual albums' pages may contain links to external stores (Play-Asia, iTunes, Amazon, etcetera), where you could find a copy of the album for sale.
Also, the website has a
marketplace, for those that feel like selling/purchasing a few albums right from other users of this community.
EDIT:
May I suggest reading
Vert1's excellent
Beginner's guide to buying VGM? That thread is useful if you plan to buy Animation-related albums, too.