Hey, would it be possible to lock the sale of items to certain oxide groups? 

I haven't thought about this before. I'm not sure if this makes sense for this plugin. This plugin's goal is to let you edit the contents of each sale offer, not to impose any restrictions. Adding scope to any plugin comes with risk, in terms of potential conflicts, and it becomes harder for the developer to holistically design the new scope with the existing scope.

Regardless of whether this feature is implemented in this plugin or in another plugin, the main usability problem your users will face is that the vanilla vending machine UI doesn't have any way to communicate whether an item is allowed. Ideally you would want to show a UI over the buy button that says "no permission" or something. You would also need to handle the case that the player managed to buy it anyway (such as via cheats) or via a marketplace terminal. In not sure if it's even possible for a plugin to detect which vending machine you have selected in the marketplace, so the UI approach may not work very well in that case.

One approach you may want to consider is to restrict an entire vending machine. Don't allow players to even open it without permission. This might also have issues with the marketplace, unless you disallow that machine from being accessible via the marketplace (not sure how to do that off the top of my head). This approach also comes with the downside of the players not being able to see what they are missing, unless you enable the map marker and they look at it.

Thanks for the reply WhiteThunder, what you're saying makes sense. I hadn't considered some of the UX needed as i would have likely implemented some of that via other plugins to help players.

In my use case, i use your other plugin Monument Addons, to spawn in an additional Vending Machine in outpost selling stuff like coffins, poker tables and ice walls etc. This is accompanied by a sign with various bits of info and of course a Pookie bear sat on top.

Locking the entire machine is also an interesting idea. I could see that being used for VIP only vending machines. This could also be highlighted with the vending machine skin. 

Though i fully understand some player confusion may still exist. Appreciate you taking the time to answer. 
All the best,