I purchased a Moni Moni red hobo from Amazon about a month ago. 2 weeks after I received it I noticed that it went on sale for about 1/2 offf. I call them and they PAed for me right away. Great customer service. The bag is authentic and sooooo soft and beautiful.
I bought a Fendi through Amazon that was listed from Jomashop. It was the same price as on their site, but since it was through Amazon I could use my gift certificates.
It took about a week to come, and it was authentic.
I think you just have to watch who the seller is - I trust Jomashop and Amazon, but not sure about any others.
I recall that another member bought a Prada bag that was being sold directly by Amazon and it was real. However, some of the folks selling through Amazon are selling fakes. As far as I know, Jomashop is legit, though.