The yellow crusted material on the dragons mouth can be anything..from a sign of a respiratory infection or the food it was feed. Pureed squash will look yellow and crusty if it dried on the side of the dragon's mouth.
However, closer examination of the dragon will be needed to learn if it is healthy. How does it behave? Is it alert, watching everything that happens around it, responsive if you move around before it's cage? Is it out basking or hiding? Are its eyes clear and open and actively looking around? What about its general appearance? Are the limbs filled out or skinny looking? What about the tail base? It should be thick and no sign of any hip bones. There are two fat deposits on a dragon's head, just behind the eyes. Are these sunken? Or level with the rest of the head or actually have little mounds?
THe one with the crusty stuff on its mouth, do you notice any fresh mucus around its mouth or nostrils? If you held it does it seem to wheeze or whistle? (it may hiss, if its nervous but typically a nervous dragon will open its mouth and hiss. If it does hiss open mouthed, check the color of the mouth. It should be a healthy pinkish color, with clear saliva but no redness, yellowish or milky mucus or cheesy like substances anywhere.
Check the vent area, it should be clean and free of any swelling, fecal matter or sores. If there is other yellow crusty matter on its body, it may have yellow fungus disease too. Now that is not something you want to worry about...
Ask questions of the store employees.. ask what they feed it, if they do feed yellow vegetables, that may explain the crustiness on the face. But if no squash, sweet potatoes or even shredded carrots, then I will be weary about getting either dragon.
It is also better to wait a few days, or look around for other sources, before buying the first pair of dragons you find as well. I also suggest if you do want to get a breeding pair, or just two dragons who you may want to breed in the future, is to say you want unrelated dragons. Even large reputable breeders may sell siblings if they don't realize the intention is to breed them.
Even small time breeders may have more than one bloodline to offer, but always good to just ask for non related dragons, to make it clear that is what you want.
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PHLdyPayne