Well my favorite game is a poker game but I need to know your service legitimately.
How can you prove regarding this matter that you are capable of your service, most likely --if you are a reputable member there someone will trust you but since you are a newbie no one will willing to avail your service, -not like if you will coach first and then payment later.
If you can give insight on this matter I will perhaps avail this.
He is a new member and he is selling stuffs but he doesn't have a red trust. I think that is still not enough to convince us all here but we need to at least see a positive feedback from those who are old and trusted members of this forum. For now the best thing that he can do is like you said teach us first before we send the payments.
We can also use an escrow service and send the payment to the escrow first and then the escrow will released it to the seller once everything goes fine so that no scamming that can happen on both sides. Poker is truly a serious game because many people offer a coaching service but the money that is spent here can possibly be recovered soon once we became really good at it.
The first suggestion will be put the seller in danger if they teach the course before the customer pay because we all anonymous here and there's a chance to not honor the agreement. It's best to do the later suggestion which is hire an escrow to purchased the service of OP.
He is selling a video course so I'm sure it's better to use shopify for sales to have a safe transaction for both party.