Whedonesque.com has some good advice as to where to find authentic autographs.
Addresses to write for autographed photos of Sarah Michelle Gellar are:
c/o ICM Artists
8942 Wilshire Blvd.
Beverly Hills, CA 90211
c/o Markham & Froggatt Ltd
4 Windmill Street
London W1T 2HZ
UK

Below are two examples of the handwriting (or printing/writing combination) of Sarah Michelle Gellar. On the left is a note written to a fan of Swans Crossing circa 1992. On the right is a thank you note written in 2010.
The Swans Crossing note comes from a fansite that may no longer exist.


Comparing the above 16 instances of Gellar's signature on photographs, one finds that the real signature seems easily identifiable by the distinctive capital letters. Below I have repeated the signatures that seem to be not at all genuine.

Finally, here is a card with the cast signatures from Buffy (the Fox years).