Blogger custom domain confusions - Check before you buy

In the Blogger community forum, I happened to help a user who was not able to publish their blogspot address to their custom domain. After doing all the debugging, there was no option left out but to contact Google and finally we have known what the problem is.

The problem was that the URL was not available for use in blogger. 

Here on, before spending money on buying a domain, please ensure blogger allows it.

Why isn't the custom domain available to buy but not available to use on Blogger?

That'd be the first question every user would ask or think to ask. Unfortunately reason is un-disclosed by Blogger themselves. 

So how do we know if a domain is available at Blogger or not?

Earlier, there used to be no way we could do this as Blogger would throw a more generic error like below,

Now, we get a more customised error for this particular case alone like below,

So, here on, before spending money on buying a domain, please try publishing your blog in the domain name you are planning to buy so that you can know for sure Blogger allows it.