Buying antique jade on eBay, especially those pieces coming from China is a losing proposition if you want real antique jade.  The majority of what is being sold out of China is in fact, modern and, in many cases, not even jade.  One easy way to figure out what is what is to look at the carving in the picture; antique jade has smooth lines, more fluid in flow, this was because it was carved by hand and the person took the time to make it artistic.  Modern reproductions are obvious when you look at curved lines because they are jagged in many cases, this is because modern tools were used, tools that jump up and down as they cut and the carver did not have the time or expertise to smooth them out.  Also when you see what looks like dirt rubbed into the cut lines, this should be a warning that the piece was "aged" to look old, as if it had been buried.  Additionally, many subjects purporting to be old simply are not Chinese in origin.  Probably the biggest giveaway would be the fact that you realistically cannot have dozens of exactly the same antique piece.  The last clue is price, as connosieurs of jade culurally, the Chinese would not sell 18th Century jade for a few dollars, if anything, they are not stupid.