Katya,
Watch youself. I have seen tons of pieces coming out of Russia which are forgeries, and there are many being made in other countries as well (the Czech Republic makes glass, China and Germany do plates, etc.)
If you do buy on ebay, make certain that the seller will allow you to return the piece for ANY reason, for a full refund.
Remember also, the services made by the Imperial Factory were huge, and were frequently enlarged. For example, the "Coronation" service (also known as the "Kremlin" service) made for Nicholas I was enlarged under ALexander II, ALexander III and Nicholas II. Pieces from the first production are far more valuable than those from the later periods.
It is important when collectiog porcelain to buy pieces which are perfect. No chips, scratches to the glaze, "firing marks", or glaze cracks should be accepted. Make certain you ask for a condition report.
Good luck!
Nick