Buying ivory antiques online are very difficult.  There are a lot of repro's out there and not everyone is honest.
First, you want to make sure that the description is consistant with the picture.  ie: date in the description matches the markings on the ivory.  Secondly, make sure there is a picture of the grain of the ivory.  When looking at the cut of the ivory make sure its not a digital cut. 
Third, look at the carving itself.  An antique carving at a close-up is inconsitant with the grove cut.  A repro is a digital cut and has a consistant grove cut.
I hope this helps.  Feel free to e-mail me if you have any questions.