Question: I am thinking of using a company that has an online panel to recruit participants for a questionnaire study. Because the demographics (age, gender, region etc) of the entire population is known (population is around 30,000), they say that they can claim representativeness of the results as they can ensure the demographics of the sample matches the population. Does this sound right? 

(sample would be at 95% confidence levels with 5% confidence interval).

Answer: 
The sample itself is not representative. But you could randomly select on demographic characteristics to meet those of the general population. Or otherwise attempt to control for your bias. In short: the selection is biased but there exists some methodologies that will help mitigate the problem.
In principle yes but there are possibilities that participants are special case where some sort of unobserved selection mechanism is at work.