I have bought several from private seller 50/50 or 70/30. I like as a buyer holding some money back that I pay before guns is release after paperwork clears. Gives buyer a reason to ensure paperwork is completed promptly.
I would be fine as a buyer doing 90/10. But you may get more buyers doing partial payment . It’s always nice to float a mg purchase over a few months.
But definitely write up a bill of sale with payment terms and break fee and both parties sign.