As Keys has suggested, watermarks are not fool 
proof.  If you you look at their Li, Sakura cel 
they just listed, they had tried to sell it 
earlier, with the small watermark still showing 
on the cel.  Look at their past auction and you 
will see "Glass Heart" in the lower right corner.

But what made me get interested in this whole 
fancel thing is that I have a chance to get this 
Spark cel, in all it's detailed glory, for only 
$30. o_O