If I was referred to a buyer off StereoNET, do I still need to mark as Sold?

Simply, yes. If someone saw your advertisement on StereoNET and passed someone else's contact information to you as an interested buyer, then yes, you SOLD the item here on StereoNET. Seller's Fees are due and payable.

It was the advertisement placed here that led directly to as successful sale to an interested party.