The best way to answer this is just to use common sense.  You want an
animal that is active and will tolerate handling.  Signs of good health can
be seen through bright, black eyes and a muscular build.  If there is any
doubt about the quality or health of the animal, don't buy it.  A federal 
license is required in the US in order to sell baby gliders, so also make sure
your breeder is an exotic pet license with the US Dept. of Agriculture [6.8].
If the breeder is not licensed, don't waste your time.