I buy really a lot from Yoox and sometimes from Bluefly.
Bluefly has excellent return policy. I bought expensive Bottega Veneta and green color was discolored a bit. I returned it without problems and bought brown BV instead which was flowless. Another case was Gryson boots that fell apart during the first wear. I returned them and got refund, no questions asked. I never had problems with clothes from Buefly.
If you doubt about the size you can chat with them and get the idea.
Though Yoox prices are unbeatable, they can send you wrong color and size, stained clothes, worn shoes (I got all of this at least once). Also their packaging is awful. They throw leather bags in the box without protecting it with dust bag or plastic bag, using only thin paper that can break easily. So leather rubs the cardbord and you get "worn out" look. Once the box was so thin, it was smashed, so was the Tod's bag inside.
Also you never know about sizing, customer care is not helpful. I do a lot of mistakes with sizes of unfamiliar European brands, so I have to return clothes quite often.
Yoox returns are usually OK, but if their stuff didn't attach so-called identification code on paper tag which happens in 10-15% of cases (only sealed rope with plastic, you know), they won't accept it, so says their return policy and customer care confirms.
And of course if you wear something and it falls apart right away there's no hope for you to return the damaged item without tag. You can get into trouble, so buy expensive items there at your risk.