I'm sorry you had to give him up, I hope that they try to at least treat him, but I doubt it...Petco doesn't quite care about their live animals unfortunately, they care about profit (I'm talking at the regional/corporate levels).
Where did you get your new beardie? I hope you did not buy another one from the same Petco location, as there's no way that they have already properly disinfected every tank that they keep reptiles in, and I'm sure they haven't yet and probably won't ever have Skin Scraping tests done on the dragons from the same litter...realize that each store gets their dragons from one single vendor, so this Yellow Fungus infection/outbreak is originating either from the vendor or that particular Petco, and no doubt every dragon from that store has Yellow Fungus, it's extremely contagious. Hopefully you got him from a different pet shop.
As far as him hissing/biting, he needs time to settle in, again he'll have a period of relocation stress that may effect his appetite, energy levels, make him constantly hide, and his temperament. How old is this dragon? I couldn't really tell if you meant this one is smaller or larger than the one from Petco, as the ones from Petco are usually around a month old at the time they arrive at the store (they sell them too young, I know the Animal Manager at my local Petco, and she makes a point of putting the new baby dragons they get in a tank that says "Just Arrived, Not For Sale")...
If he's an older dragon that has never been handled or had much attention, then it can take some time to get him used to you. If he's another young baby then it's most likely just some relocation stress, but please be sure to check all 3 temperature zones inside his new tank, make sure that you have an adequate, strong enough UVB tube that is not obstructed by a mesh lid or clear plastic cover on the fixture, and that the UVB tube is mounted within the correct distance of his basking spot/platform. Inadequate
UVB light, improper temperatures, bad UVB lighting (for example, a Coil/Spiral
UVB light that hurts their eyes) can cause them to feel badly or stressed out. So just double check that all of your lighting and the temperatures are appropriate for a dragon and set up correctly for him. Hopefully you are using either a Digital Probe Thermometer or an Infrared Temperature Gun to check his temperatures, as you can't measure the Basking Spot Temperature at all with any type of stick-on thermometer, they only measure Ambient/Air temperatures and not Surface temps.
Otherwise, just give him time to settle in, and talk to him often and gently, let him get used to your voice, and always approach him with your hand from the side or from underneath, allowing him to see your hand before you touch him (never approach from above where he can't see your hand). A lot of people will tell you to not touch your new dragon at all, just leave him alone for the first 2 weeks, but I've never agreed with that at all, I actually believe the opposite. It's a subjective process, but I'm a believer that having a new baby dragon with you/on you often will get him used to you quickly. Once he realizes that you're not a threat to him but rather his new family, he'll warm right up to you. Again, I don't know how old he is or how large he is, but a baby or juvenile dragon bite won't hurt you, it's usually just a shock or surprise, but they don't hurt. So if you can handle a little bite or two, usually once you get them out of the tank and on your lap, sitting on the couch or on the floor (where you can't accidentally drop him or have him suddenly jump off of you from a distance and land on the floor to surely be injured) and then just talking to him softly, gently, usually they'll allow you to stroke their head, ears, and back, they realize it feels good, that you're not a threat, and after a few days to a week they tend to settle in. It just takes time.
I'm hoping he's not an older dragon that was owned by someone else already and he's hissing and snapping at you, because that's a sign that the dragon was typically just neglected and ignored, and that the prior owner never handled them or paid any attention to them at all. This takes a bit longer to warm them up to you, but it still happens.
