What Trojan said. I have about three different sizes in my closet for biking jerseys, just depending on the vendor and the cut. The one time I ordered cycling tights online they were a bit small, albeit serviceable, and the only reason I did so is they were too good of a deal to pass up. I'd recommend Nashbar if you insist, they generally have quite a few sales that it can make the gamble more worthwhile. I'd stay far away from eBay, unless you find someone dead reliable, most of the Chinese junk on there anymore isn't even worth the cut rate prices they ask.
Once you know what brands and sizing in them you like, it isn't so bad, but until you sort it out it can be a gamble.