I have used Ebay for years and years. I usually see something that's too expensive for me in-store, note the size, fabric, measurements, then stalk Ebay until I find it for cheaper. This is the most reliable way to do it, for the reasons that Becky said. I've never been unhappy with an item I've bought this way (jackets, shoes, many pairs of jeans).
I also regularly buy brands I know well and have experience with. I know, for example, that I take an XS in Ben Sherman blazers, but for some reason need a S in all their shirts... if you know a brand well, then you're less likely to be disappointed.
The best way to minimize error when buying "blind" is to ask for lots of very specific information. For example, I learned that, for my jeans tastes, it's not enough to ask if it has "stretch" -- I have to ask the exact fabric content to know how much spandex, lycra, whatever is in the garment. When I want the shoulder measurements, I ask them to measure "across the top, from seam to seam" instead of simply asking for "the shoulder measurements." For boot height, I ask them to measure "from the back of the heel to the top of the shaft, NOT including the sole." For shirts, I ask for the measurement "from armpit to armpit seam." This helps me to know that they are actually measuring the dimensions that matter to me. I also commonly ask the seller what size she wears, and if the item would fit my bra size. A great seller will be happy to answer your questions -- silence and difficult communication, however, is always a bad sign. OH! Also ask if it has been through the dryer -- I've been the victim of shrunken jeans before.
Yikes, I guess I wrote a lot... sorry for the dissertation, but I hope it helps!
