Most breeders sell at 6" or more or at least 6weeks of age. Right out of the egg babies are far more likely to die if sold right away as the stress of moving and being in a new environment could do them in fast.
Feed babies about 2-3 days after they hatch..though its fine to offer food after hatching. Most just don't eat right away due to being 'full' from absorbing the last of the yolk just before or after the initial pipping of the eggs. Definitely get alot of small crickets, 1/8th to 1/4" in size. Once baby dragons start eating they can eat as many as 100 crickets a day spread over 3-5 feedings.
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PHLdyPayne