Ok, does this thermometer have probes? What I would consider doing is to remove it (if you're able to) from the side and place it on the inside of the vivarium, under the bulb where the beardy basks. This will help you find out the basking temperature. From there, if your tank is of a good size, the thermogradient should follow. Obviously do this later as your lights will probably all be off by now. You want the basking temperature to be 105-115F depending on what your dragon prefers.
If you're able, describe the rest of the setup (or pictures). Things such as UVB lights (brand, age, percentage), size of tank, usual diet and supplements are important to mention. The breeder sounds like they have unorthodox husbandry, which won't be helping your beardy. Also the lady at the pet shop should really have recommended a thermostat.
The problem could potentially be husbandry related, difficult to say though. I would recommend getting the dimming thermostat and another digital thermometer (preferably with probes) to begin with. This will stabilise the temperatures in the tank automatically so that he doesn't overheat or get too cool. Overheating is really quite dangerous for any reptile so you want to get that heating sorted pronto
I understand that there's so much conflicting advice and information around, so can't blame you for getting confused!