His results are actually misleading. If you actually look at his merchandise had a hike the week before.
Here is my merch records and player sales.
https://imgur.com/a/9luJyI1As you can see, I sold a trainee I drafted on 1st Feb after the financial update, and got a 2k drop in merch sales the following financial update. The thing is I was tanking and my merch has been steadily declining. On 10th February, I sold another trainee I drafted, and had a 2k rise in merch on the following financial update. On 25th March I sold off all 3 of my newly drafted trainees, and the following update (29th March) I had a 8k boost in merch.
Selling trainees and bit part players should have too little effect in merch. Any change in the merch is probably not due to it.