Simple Delivery is mandatory for private sellers, only business sellers don't use it.
Heads up that if you're selling items for your mates they'll still count as sales info that eBay will send to HMRC once you sell over 30 items, and HMRC may consider you to be a trading assistant if they're not your items and you're giving payments to your mates. I'd recommend that they open their own accounts to sell their items on, you don't want to be landed with their tax bills.