I use it a lot because I'm a canuck, and you know our sore state of shipping/purchasing up here.
I use it to stalk things. For example, one of my saved searches are ... "Anthropologie Notched Dots Dress 4" and if they get one, they email me about it Yay!
I am very careful about ratings, and the quality of the item (I need to have actual pictures of the item, not stock photos).
One way you can get really cheap good stuff is ... if you have a lot of time, just browse. A lot of the great deals I've gotten (>80% off) is things that people have mislabelled or spelled incorrectly - For example, a Fossil Weekender Navy could go for $148 to $220, depending, but if someone called it a "Fossil Navy Satchel" it's impossible to find unless your'e just browsing Fossil stuff - the Weekender crowd wont' see it, and I snagged it for $40. New. From Vegas. Whoooo!
For sizing, I know my measurements and always request them! I have had good luck, and if things are a smidgen too big, that's fine, I can get it tailored.