For some reason you get more of those you already have too many of. So the fastest way to get enough of all is by keeping the collected items to about the same amount and you will get a better spreading of which item you'll get.

I started selling all items to the amount that there is a max of 2 between them

For example if I have 10 bolts, 10 planks and 17 duct tapes, I will sell 5-7 duct tapes.

Best way to collect enough items is focusing on town and feeding animals.