(b) That's right. It would be commercially impractical to insist that goods could only be bought and sold by a person who is already the owner of them. It is quite common for a seller to bind themselves by contract to sell goods that they do not yet own but which they intend to acquire in order to fulfill their obligations to the buyer. From the buyer's point of view, this is not a problem, as long as the seller is subsequently able to acquire the goods and make the buyer the owner of them. To guarantee that the buyer gets good title, the relevant sale of goods legislation makes it a condition of the contract that the seller will acquire the rights of ownership and transfer them to the buyer by the time that the right of ownership (title) is intended to pass to the buyer.  Until then, the seller need not be the owner of whatever is being sold by them.

The sale of goods legislation makes it a warranty of the contract that the buyer's possession of the goods will not be disturbed by a third party who has a superior claim to ownership (this is called the 'warranty of quiet possession'). The legislation also makes it a warranty of the contract that goods bought and sold are not burdened by any undisclosed charge or encumbrance in favour of a third party (such as when goods sold are subject to a mortgage created by the seller in favour of a third party).   See a list of the relevant sections.