What proof do you have that it was sold? It's been transferred to another learner, but the £500 - £1000 figure appears to have just been made up.
I agree the figure is made up, but it's based on what I have heard instructors or scalper websites quote for an "early" appointment when DVSA doesn't show any availability officially. Maybe the figure is £250 for two appointments, I don't know, but that's not the point.
This instructor risked so much by swapping the slot (NOBODY else knew the address, license number AND the appointment confirmation number besides him) so I suspected he must have found it "worth it". Especially when he has been paid more than £1000 cumulatively for classes since January this year.
Have PROOF emails for all information shared with him and have proof of payments made to him for classes. Also have DVSA confirmation of the slot swapped on phone. Of course NO proof of the exact amount he earned in the process.