Originally Posted by
miikkak
In practice it's very complicated if one isn't BAEC member. Collecting the fare difference requires that they have enough information (address etc) to be able to send the bill which they often won't have if tickets are bought through OTA and paid through online banking.
Not really, when booked through a ta (online or not), airline just sends a debit memo to said ta. It is in fact the most common recovery case. Individual travellers are more problematic as it's hard to force them to pay and taking to court has reputational risks.