Views contextual filters by node title
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.