well there are plenty of sites overing infomation on weather certain items are Celluloid, french ivory or the Real Mcoy, the down side is the way to tell most of these is by finding out what it smells like when melted slightly, but there is also another way to tes once you have the itemwithout destroying the product and that is to use a Convex or Concave sheet of glass you should see Green grain or dots. Check
HERE For details, maybe you could ask the Seller to take a picture through a sheet of glass before buying an item?