Personally I would avoid it. The reason being is that the seller is saying that the breeding happens by natural process, the only proven coturnix-chicken hybirds I have seen have been inseminated. All he has to do is not guarantee the hatch due to shipping concerns and send you any old coturnix egg since he supposedly has an easter egger bantam roo covering a coturnix hen. Even if he is getting hybrids by natural breeding there is no way the process is safe/humane for the quail, and that is leaving out the fact that chickens carry diseases that are often fatal to quail.
Here is some reading on the insemination subject. The fertility rate of inseminated eggs was low. The hatch rate was even lower. And this guy is selling "hybrids" that supposedly happen through natural cover? I'm skeptical to say the least.