Amazon.com, overstock.com, madisonavemall.com

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.