I don’t buy parts unless I need them. I do keep nuts, bolts, and washers handy in 1/4, 5/16, and 3/8 inch, some 1/2 inch. 90% of my needs for bolts are satisfied with that assortment. I also keep wood screws and some small machine screws handy. I also keep some angle and flat steel around for fabrication of parts. I also have round stock for my lathe. I try to keep all of this to a minimum.