The blue personal boxes in Skadovsk, Yanov, and Pripyat are all safe places to store stuff. If you store loot in other boxes it's possible someone will steal it.
You will find the game to be much more tedious if you hoard everything. The whole point of STALKER is to carry only what you need. That means 2 guns, 3 at the most, ammo, some medical supplies, and maybe one or two extra artifacts. You can get artifacts and upgrades to increase your carry weight but using one of those artifacts prevents you from using one with more helpful effects.
Starting a few hours in, enemies will start to drop lots and lots of assault rifles. They'll all be too damaged to sell, but you can either repair them and sell them (you can still turn a profit, at least near the end of the game once you've gotten discounts from all the mechanics) or you can go to Pripyat, repair them for free, and then travel back to an area where you can sell them.
I can only imagine how horribly overcrowded your personal box(es) would be if you didn't actually sell anything... dear lord. You'd have like 20-30 each of L85s, AR15s, AK47s, AK74s, AK74us, various shotguns, not to mention all the pistols. That's a lot of back-and-forth trips.