You self supposed know say nobody wey get enough sense go gree trade such physical trade even though say the rate too good except say na person you know already offline.
Imagine saying you want trade a $1000 dollar to naira with the current price na very big risk to carry such money self dey waka for both the vendor and the seller.
Knowing the person is not enough reason to trade your coins for physical cash, what's stopping him from sending the money through bank transaction if the money is legal. Most of the vendors offering physical cash trade for your coins are kidnappers, Money launderers trying to wash their money clean.