Days into the apocalypse, staples and basic items start to disappear. Gasoline, sewing needles, shaving razors, thread, coffee, alcohol, sugar, flour, matches, lighter flints, ammunition, and later on, salt.
Several months in, the lack of salt turns desperate, with a salt starvation setting in amongst the population of Fort Repose.
"Since July, he had been unable to trade for salt anywhere. In Marines Park, a pound of salt would be worth 5 pounds of coffee, if anyone had coffee."
The lesson: stock up on these items while they are cheap and readily available.
Small, cheap, non perishable items are the easiest to stock up on. Bic Lighters, cartons of matches and packs of lighter flints are a buck or two at the grocery store. Sewing needles, thread, bottles of rubbing alcohol, cotton balls, petroleum jelly, hand sanitizer, N95 masks, razors, screws, nails, safety pins, etc. are similarly inexpensive. These items will not go bad, will be difficult/impossible to replace post-collapse, and are useful if things never goes south.
Bulk supplies of food stuffs should be kept for your family's use and generally not barter unless you have excess storage room. Salt is one exception--50 pounds of table salt is around $8-$10 at Costco, and boxes of Morton table salt are usually around $1 at the store. It lasts forever, and has a multitude of uses beyond simple seasoning--preserving meat, saline solutions, and more. People generally only have single box of salt at home, so it will be in high demand quickly post-collapse.
Don't go nuts stocking up barter supplies for a possible TEOTWAWKI survival scenario. Don't fill your home with useless crap. Don't waste money--buy things that you will use if the world doesn't end, buy things that are on sale, use coupons, hit the dollar store, and so on. But $10-$20 a month spent wisely on inexpensive, compact hard goods adds up quickly. After six months to a year, you will have a sizable cache of goods on hand, for use in good times and bad.