From a very brief look at some of their listings, the site seems pretty genuine and well set up to me. Horribly overpriced stuff (again only from my brief look) but nobody fakes modern postage fodder for the secondary market. It's not worth the effort. They are just listing stuff at overpriced prices (from my brief look).
The stamps are 99.999999% likely to be genuine, the prices, well, it's up to you.
jyunny, could you please post on the welcome forum as you were requested to do in the eMail that enabled your account.
Geelong, VFA Premiers 1878, 1879, 1800, 1882, 1883, 1884, 1886, AFL Premiers 1925, 1931, 1937, 1951, 1952, 1963, 2007, 2009, 2011, .