Common items, keep quantity on hand you use 30 to 50% of stash per year. I keep 1 to 5 lbs each common drywall screws, maybe 10 sizes total. Keep jar each size hand pound nails; 10 pounds framing & sheathing nails. Maybe 50 cans different color/type paint, stains, varnish. About 20 sheets each grit sandpaper, from 50 to 3000. Full assortments all sizes pop rivets, rivet nuts. 5 lbs each type weld rod. 20 types glue. 10 types shop oils. Quart to gallon each type solvent; alcohol, thinner, naptha, turpentine, toluene, acetone, MEK, zylene, goof off, kroil, WD-40, kroil, etc. Full assortment Kreg screws. Full assortment sheet metal screws. Pipe dope, antisieze, teflon tape, thread locker, etc. Full range colors electrical tape, double stick tape, duct tape, paper tape, aluminum tape, reflective tape, etc. Half dozen types caulking tubes. 50 types wire, I do much electrical work so use lots. Full assortment terminals, wire nuts, crimps, etc. Full range metal prep and protection products; brasso, rubbing compound, plastic polish, glaze, wax, evaporust, etc. Scotchguard, Rain-X, Armorall, etc. Keep 2000+ dremel tips, buffs, sanding sleeves, etc; use many, many year. Keep oil topoff cars, brake fluid, wiper fluid, fuses, power steering, etc.
Uncommon stuff, keep range on hand allows me do all stuff want do. Me, keep about 1000 small drawers full fasteners, small hardware. Only a couple bin boxes bolts, maybe 60 bins. This save me running store when need odd thing; but need more bolts, go buy there. Was into hardware cabinets 3 or 4 times today, used impossible get hardware store stuff twice. That why have the uncommon stuff; necessary to have.
Building materials; keep absolute minimum; buy each job. Take more space than worth store long term.
I buy lots hardware estates. I scrapped 40 to 50 small drawer cabinets full so far this year. Every estate have 5 to 10 small drawer cabinets full; mostly unsorted leftover junk. I only keep brand new, clean, sorted stuff. Preferably large quantities same stuff. Dirty, rusty, unsorted, gets dumped back into drawer cabinets; I resell full cabinets to dreamers/hoarders. Coffee cans or jars stuff, not worth time to sort; it go away again.
Junk most people keep in garage that I get estates, I fill boxes; once year put out in driveway; have yardsale everything 25 cents; people dig through get 30, 40 dollars worth at a time to take home fill their garages! Sold over $750 worth this years sale. When done, put ad up and lady came took pickup load leftovers away.
Bottom line, keep stuff you use commonly, keep sorted well, never keep rusty/dirty, if can get hardware store easily and not used up in next few months, get rid of it and buy when needed.