overstock.com sells authentic merchandise! I was able to get some awesome deals on Fendi scarves from them. I can't comment on the others, as I'm not as knowledgeable about these sites.
i wouldn't trust amazon b/c i think it's regular people that sell stuff, kind of like e-bay but not auction style. overstock has had some iffy stuff about them, some items that were not authentic and some that were, do a search on that.
and i haven't hard about the other site but hopefully someone can tell you more a bout it.
I can only speak for myself, but I received a fake Fendi Spy from Overstock. I had it authenticated on this forum and thankfully I was able to return it.