Instead of buying things to resell, I prefer to buy in advance things I'm going to use. For example enough concentrated laundry soap for 1 year, or tuna cans for two months. They have a good volume to price ratio, and they don't spoil. [I could boy more tuna cans, but it looks too weird.]

This is useful to absorb extra payments, for example we have a bonus half salary in July and December.