I mostly make my purchases on the long, indepth reviews not only on this site but all over the internet as I feel I can trust them. Also, how popular the album and artist is also effects my purchases.
If an album is popular then it must be good but then it might not suit your taste. If its not well mabe not so good but i give it a chance and read the reviews if its any good.
Then I listen to samples on saikosounds. Usually the first three songs, if I have time then I listen to all. Then I buy it, if i could find it.
Hope that was useful.