I have tried it and it's a scam.

I have used it for a small amount (0.00698730) and after my transaction for confirmed, order details just got deleted. No letter of guarantee, nothing. There was one page with order details and that it's deleted right now.

This is my transaction: https://blockstream.info/tx/0b4f2ef4dade222494aa6ee24d617edf9ad6e35976c071deae7cbe6cdcc4227f

I see that more payments were made to the same address. It looks like he is switching a few Bitcoin addresses daily. I don't know how nobody complained before.

Good thing is that this is not a big deal for me, but I will make him pay more then that.