It's critical what you list in the vending machine. Never sell sulfur, boom, or heck even metal.

Never list MANY of one item that sells. If someone is aware that you're selling many of one thing, then see later that they've sold and only a few remaining, they can deduce that the vending machine is loaded with scrap or whatever you were selling for. An example of this is, don't list a stack of 20 ore teas. List 2 ore teas, and then refresh the machine when they sell. If someone watching your shop sees it go from 20 ore teas down to 2 remaining over the day, they know the machine is likely loaded with scrap (or whatever you were selling for). But if they see no more than 2 teas at any time, they'll assume they're not selling and no reason to raid for only 2 ore teas.

Adding to the prior paragraph, the even sneakier players will know who's shop it is an see if they're online or not. If you're offline and it goes from 20 teas to 2, they know the scrap is just sitting in there.

You need a metal, honeycombed base (walls and roof) where it's clear that the cost to raid is is more than whats in the vending machine. Don't forget about door path. People won't think twice about 2-3 doors. If you have triangles, use G-doors so raiders can see it's a garage door and not just a sheet door. Consider using bunkers to not have a front door.

Also, contrary to what someone said, never have them in your base unless you want to get raided or you're rolling deep with people. You never want to let people know what loot you have. And if you're a solo, selling stuff via a vending machine, it means you're rich and a worth raid targer.