In general, the more expensive the line, the smaller the sizing runs (Brit largest, London next, then Prorsum the narrowest). However, there are definitely aberrations and, like Anna said, it can be frustrating. I usually wear a 2-4 in jackets, xs in Tahari, US 6 in Karen Millen, Italian 38. My black trench is a 4, while my green one is a 6.
If you plan to buy on Ebay, my best advice would be to try on as many styles in store as possible to gauge the fit. Then you can search Ebay for the style that you like. Or you can reverse engineer things and see what is available on Ebay, then do sizing in store. Double check by asking sellers for measurements. If you can't find the available Ebay style in store, the Burberry people are excellent resources. Email customer service or call a local store. That's what I did for my green one... CS gave me the # to a store with it in stock, and the SA there tried the coat on and helped me decide on sizing.
Either way, in store research helps you in two ways when dealing with Ebay: you know what to expect regarding fit, measurements, fabric drape to make sure you like it... you also know what to expect to make sure it's real. If the measurements are wonky and it's the wrong material, you know to run the other way.