The purpose is to give Bob proof that Al has enough coins to complete the transaction, and to be sure that he won't spend them.
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My idea sounded good on paper, but it is apparently very difficult to implement because of the way Bitcoin works (and is probably not the best solution).
The "problem" is the same problem you have when selling your house for fiat money. Here you'd use a notary as middle man, to ensure both parties fulfill their end of the deal.
If there's a breach of contract, one party can sue the other party for compensation.
Changing the payment system doesn't change the other requirements. You may need to explain the origin of your funds to the notary though.