Hi Matt - with respect, I don't think its quite as simple as this, and thousands of pages and centuries of jurisprudence in the extraordinarily complex area of contract law would point to it not being so simple.
Your example of purchasing milk is an example of a simple transaction where title in the property passes at a clear and defined point, you pay and leave with the milk, both parties are under an understanding that at that point the ownership and title passes - however lets say I ordered my milk & groceries online with Tesco, Importantly, I pay up front and they deliver later that day, at what point do the groceries become mine? when does title in the goods pass? when I pay, or when its delivered? what if its delivered and its the wrong item? what if the ice cream has melted - does risk in the property transfer from buyer to seller at the time of payment or only receipt and inspection?
What if a transaction was illegal, it could easily be argued that no purchase has actually taken place, since such transaction was illegal - if I sell you something I have stolen, then title cannot be transferred, since I did not have the power in law to deliver that title to you.
Again, your example of the sweets is a messy one - if its illegal to sell them on sunday, but I pay you on the saturday and you deliver them on the sunday, was the purchase legal or illegal, conversely I pay you on sunday and you deliver monday, legal or illegal - If I do my shopping online and order alcohol at 6 AM on a sunday morning, was this illegal because the purchase took place outside of licensing hours, or is it ok because delivery only took place inside them?
Its a complex area of law, there's whole books like Chitty on contracts written on this and I, respectfully, don't think you can dismiss what constitutes a purchase as being as simple as buying a bottle of milk!