I don't know about any websites but I have alot of Seraphon and even some WHFB bits I could sell, assuming you are US based.
Apart from regular auction websites I can't recommend anything, but you can try local hobby shop where people play the tabletop that is not an official GW one. Usually there is a lot of people willing to sell something including the shop owner.
I looked a bit into it, but there ain't much different factions of AoS available. And it is pretty expansive: 5 Drakespawn Cold Ones (only the cold ones) will cost you 40-something euro's, while they go for 26 euro's/box on the GW-site. So be sure to double check before ordering Gr, Imrahil
It seems to come and go with new armies going in and out of fashion. Useful for lots of other stuff though, it used to have a huge reaper stock.