Only fungi I know that can attack bearded dragons is yellow fungi but it normally appears around the vent area first then onto the body.
From what you describe on that bearded dragon at the petstore, sounds more like mouth rot or infection of wounds around the month. The pet store should take the dragon to a vet to be properly taken care of instead of trying to fostering it off on a customer.
If you do take him be aware you will have to practice strict quarantine measures to ensure no cross contamination with your dragon (it may not be contagious but always good to quarantine new animals anyway). So separate cage in a separate room with his own feeding dishes washed separately from the other dragon's dishes etc. No feeding crickets or other insects the sick dragon didn't eat to your healthy dragon or putting those insects back in with your other insects (maybe overkill but why risk it?)
Then make sure you have the funds to pay for a vet checkup and treatment (if its fungal or bacteria infection, Vet may have to do a culture to identify which fungus/bacteria it is and give the proper treatment) blood tests etc may also be needed. This may cost a few hundred dollars to a thousand or more...depending on what is the underlying cause of the mouth problem. Right now without even a picture, it can be several things. Mouth Rot, nose rub that has become infected (sores caused by rubbing his nose against the walls of its cage till the skin is worn off and becomes infected or scabs over), unshed skin, bite injuries, even burns could be what you describe. Heck, could just be caked on dirt/food. The fact he eats well could rule out mouth rot which often starts inside the mouth and is painful, but it could still be infection...and if the pet store is just saying he eats well, doesn't mean he really does, unless you have seen it gobble up insects and greens.
-----
PHLdyPayne
