if there was a reliable way to track and prove irrevocably that the BTC had ben sent to the buyer [but] with BTC addresses being nothing but a random number of letters and numbers it's impossible.
It is not impossible. It is fairly simple and straight forward, actually. You are describing a standard contract.
The contract between buyer and seller defines payment terms and simply says that delivery of the bitcoins is completed when N BTC are sent to the Bitcoin address X and all transaction(s) in which the coins were sent n have N confirmations.
This contract can be signed (physically, with a pen) and sent by mail, fax, e-mail attachment, whatever method is legally recognized in the two jurisdictions. In certain jurisdictions, a digital signature (e.g. GPG) is acceptable as well.
It has been accepted that Bitcoin would benefit with the inclusion of a payment protocol which would lessen the need for an external contract like this, however that protocol has not yet been implemented.
If you are looking for escrow, there are several listed here:Escrow list