I've been buying from Yoox for over 6 years, until this year from Europe but also during this year from here in Chicago. I've bought the majority of my Furla collection from there, as well as a few pairs of Diesel jeans, and I've never had a problem, except for the one time a bag was badly packed, as it was the last one they gave me a 15% price adjustment and the bag was still perfectly fine.
The reason the prices are so low is that Yoox is an Italian site, and the majority of the designers they sell are also Italian (Furla, Diesel etc!) so they are buying the items locally and getting a better deal. Outlet malls and discount stores such as TJ Maxx are still fairly uncommon in Europe so Yoox is the first port of call for most of these designer's unsold items. Almost everything on Yoox is at least last season, but often the designers aren't well known enough here in the US for that to matter.
Their return policy here in the US is less favorable than it is in Europe but they are still very easy to work with (restocking fee of $9 if I remember correctly and no refund of any shipping fees paid). They ship UPS with tracking although it takes 5-7 days in my experience as they first ship to their NJ warehouse from Italy and then out to the US address.
The items are absolutely authentic anyway, and as much as it pains me to let out my secret source, it's a great place to get a lot of great bargains! I would and have recommended Yoox to many friends and colleagues with no complaints.
