Oh, I usually think stocking up is a bad idea - it leads to meals that are less nutritious and tasty, and it's easy to waste food that way.

When I meant buy what is discounted I didn't mean buy more than you are going to use, I was talking more about not planning to make Thai curry and then paying extra $$ for bell peppers out of season, instead see what is discounted at the store - maybe turnips or rutabagas, and then make your meal plan based on that - e.g. pasties, baked winter vegetables, or a soup instead.

The point I was trying to make is that pre-determining the ingredients you need to buy will end up costing you money rather than making the meals based on what is currently discounted and cheap.

Regarding back-stock: you have good tips about extending the life of rice or beans, storing in mason jars also help reduce the likelihood of pests getting into the food. Oxygen absorbers are a good idea, too. However, if you are poor it can be a lot to buy mason jars - so saving glass like used pasta jars, or seeing if there are any free or cheap jars through Freecycle, Craigslist, etc. might be an option.

Though I will say dry beans don't last as long as I originally thought, and whenever I stocked up on dry beans I found they were really hard to get soft when I eventually used them, even after cooking for more than 24 hours and soaking the night before. Old beans just stay hard. It's better to treat beans as something you buy fresh, something that lasts months rather than years.

Similar issue with brown rice, or anything that isn't super refined (e.g. whole wheat flour) - you want to buy those in small amounts and use them quickly, they are more likely to spoil in a short period and lose nutritional value and flavor.

Refined foods like white rice are easier to store for a long time, and I keep my all purpose flour in the freezer so I can have more on hand than just what I'm using - that works pretty well.

You can also buy whole wheat berries, then make flour from scratch, but when you're poor it's hard to have time for all that.

Canned foods are more expensive than buying dry stuff, though it is a way to always have some extra beans on hand that won't go bad as quickly as dry beans, so if you have the extra cash it's not a bad idea to buy some canned food, if you really will use it. Just don't buy a lot - one or two cans and rotate them as you use them.

EDIT: to be honest, when I was poor I didn't have the energy to cook or optimize my grocery habits. Mostly I ate whatever food waste was in the kitchen I was working at, and some days that meant I mostly ate a bunch of white rice and nothing else. My diet was fucked up and it wasn't healthy, but it meant I rarely spent money on food. Being poor also meant I spent the money I would have spent on food mostly on beer so I could relax in the evening. I mostly cooked on weekends when I wasn't working, and I mostly bought whatever sounded good like steak, but from places like Walmart. Poverty unfortunately makes it hard to optimize your life, as you rarely have the time or energy, let alone the will power or mental health, to be efficient.

Getting out of poverty ironically gave me more energy and leisure time to spend on cooking at home and so on. It also meant being able to escape living in ghetto apartments and moving into suburbs where I could grow food in a lawn, e.g. I didn't have to buy garlic for an entire year just based on what I grew in one season - that's something I could never do living in apartments.

All this to say, if you're poor just survive and find a way out, don't beat yourself up for being inefficient if you just can't be.