How hard is it…

[Protagoras]

Looking at the on-line application process, it requires one to enter the number of a current licence.
But apparently the process allows for that to be changed once booked; what use is that?

So presumably, an 'agent' only needs some valid driving license numbers to get through the process and reserve appointments.

Presumably there was no requirement to prevent appointment re-selling when designing and testing the system?

Perhaps making appointments only available to the original licence holder would deal with the problem.

This problem is well known, and has been in the national press.
https://www.theguardian.com/uk-news/2023/sep/03/an-absolute-mess-learner-drivers-forced-to-buy-tests-on-black-market-as-companies-block-book-slots#:

[hobnob]

I know someone who took a driving test recently. They booked on for the first one they could find (just to get registered on the system), then their instructor was able to transfer it to a better place/date. They didn't have to pay any extra charge for that, so that's different from the "scalper" behaviour that you're talking about.

However, it might be more complicated if your relative is booking on their own rather than going through a driving school.

[sreed]

…for the DVLA online booking system to stop agents from booking up and reselling driving test slots?

Trying to help a relative book a driving test. There’s been nothing available on their system (for the whole 24 week window) but apparently there are folk running WhatsApp channels selling test dates online for a pretty penny.