How to package PCs to survive shipping

As my side job I buy and sell used gaming PCs on online marketplaces. In my country, many people don't want to ship their PC via post, so I often send the packaging material to the seller and take responsibility for the shipment. However GPUs are a huge issue. They very often break during shipping and then I'm left with a huge loss.

The tricky part is that I can't rely on the seller to do anything technical. They're regular people, not PC enthusiasts. So whatever solution I send them needs to be very simple to install.

The major constraint is that case size and GPU position vary significantly between PCs so it is really hard to find a universal solution.

I previously used Instapak foams, but apparently they are illegal in consumer use and I am scared of getting in trouble so I stopped using them. Otherwise they were great.

Does anyone have any ideas? What do PC refurbishers or small shops actually use?
Thanks in advance!