I started playing EFT about a month ago, and I am still a noob myself, but I'll give you 3 basic tips that will help you to accumulate money.
If it has a purple background it is valuable. Put it in your secure container.
Ammo can be worth a lot of money and takes just 1 slot. If unsure, keep ammo which has BP or AP or RIP in its name, or if it begins with M, like M995.
Check this image (not mine): https://i.imgur.com/V4BCd71.jpg You will need those items for quests. If they have green background in the image, they need to be found in a raid, no matter before or after the quest was started. So you can start hoarding from lvl1. Keep those cigarettes, you will need 21 packs of them! Also, crafted items for some reason count as "found in a raid", but you can't craft everything.