Views contextual filters by node title I've created a content type: seller and with pathauto module I've set this content type to be seller/[node:title] so every new seller content will be www.example.com/seller/[name_of_the_seller].

I want to add gmap objects in every seller page with only their objects will be visible. (for this detail I'll open a new thread, 'cos I'm not sure how to filter that)

With Views module I've added a page with url:  /seller/%

and in contextual filters I've added Content: Nid.
In When the filter value is NOT in the URL I've selected Provide default value and in the field bellow I've put seller.
That didn't worked, so I've tried with Raw value from URL and checked Use path alias.
That didn't worked too. I've searched couple of other threads, but I didn't find like my problem.  

UPDATE

I've solved the problem by creating a block that is displayed seller/*. The two content types has a same taxonomy terms and with contextual filters  Content: Has taxonomy term ID, Provide default value, Taxonomy term ID from URL, I've marked Load default filter from node page, that's good for related taxonomy blocks and Limit terms by vocabulary and I've selected the seller taxonomy.
Since your contextual filter is expecting an nid (you said your contextual filter is set up as 'Content: Nid') - you really have to provide it with an nid even when that filter is not present in the URL.  You can't give it 'seller' as an argument or anything like that because that's not an nid, so it won't help it pull up the correct node.

So to get it to work in those cases, what you really want for your view to do is grab the nid of the current node it appears on.  It's OK if that page has a custom URL, the nid can still be gotten.

Just select 'Content ID from URL' as the default value when an URL argument doesn't exist!  That'll work even if it has a custom alias for the URL.