If he has a full wallet, it may work, but that would mean the buyer comes with his laptop (why would he do that if all he needs for checking is a browser which he already have on the smartphone?)
This it the detail I was missing in my description. Of course he has to come with his laptop or with his mobile wallet that doesn't have internet connection.
I meant that people don't really have full wallet on smartphone. On smartphone they'll have a SPV.
And of course he doesn't have to check any other thing than his own full wallet.
Well, Blockchain.com can be faked even easier, after all, but as I said, he will have to know what (websites, servers, ...) to fake, there are too many options.