You can sell your own knives as a Gold member. To sell someone else's knives, you need to be a Dealer.
SECTION 1- Membership Levels Defined, and
SECTION 3 - Rules for Sellers / Traders / Service Providers at
6. No sales for third parties. If your "friend" wants an item sold, they must create their own account and list it themselves.
So if you are going to be the middleman for them, you need a Dealer membership.