Indeed, fast-transfers are just an Authority to accept a sales contract and action on the sellers behalf. but even these don't give you as the buyer, any additional contractual rights if they don't go through, apart from a refund of monies paid that is.
This applies to all sales, whether it be a vehicle a house or a domain. An advertised price is just an indicator of what the seller may or may not accept. Now that said, In some instances a Sales platform can make a contractual obligation between themselves and a seller but that doesn't include you as a buyer in those rights.
Honestly, it isn't as complicated as it reads when you let common-sense and logic dictate the grounds for any transaction. Are these rights and tribulations always fair, No not always, but the alternatives or amendments would create far worse scenarios.
If you love the subject of Business Law, as I do, you can get into 'Specific-performance' Liabilities, penalties and a whole lot more, It's great fun. You can't forget anything you learn purely because it all comes together and dare I say adds-up.