I've been wondering this, since I might sell/trade/auction some. I'd like to know what they're classified as so I know what quantity to auction them in. And so I know for stuff in the future. I'd also like to know around what 1 Zombie Virus can sell for, if you can answer that.
You can auction off Zombie Virus in DC quantities. They are custom items, but not promos. Promos are/were available by /promo and usually available to buy at /shop at the same time (unlike the 2014 cake around now).