Well fortunately for you, I wrote a pretty comprehensive noobie guide in this very forum! Read through it and you might (or might not) pick up a few things. One section deals with getting the settings right.
You may or may not be happy to know that in 13.9, they have defaulted the buy settings to disable. So future people wont have this problems so much.
I recommend allowing NPC store buying though. If you disable it, you are unable to buy cheap fruit, soda water, wad of dough from bugbear, etc. This means your item manager will be rather limited in being able to create food and drinks for you. No NPC item that you would need would really spend a significant amount of meat.
I can see a button being handy as I sometimes enable meat buying from the mall temporarily for some reason.
You might be able to talk one of the people here into making a button for you. Under Preferences, Shortcuts tab you can define buttons yourself. All you have to do is have it cli_execute something to change that particular setting, or do an ash script to Disable buying if enabled and vice versa.
I dont know which property this would be however and the buttons you define are only visable in some odd locations. (minibrowser and a few others)