Assuming the prices are low (not always the case), I would say that the best buys are the most humble, ordinary items. The savings are perhaps low, since the items are not expensive to begin with, but over a lifetime...I was going to say "Do the math," but in this case I'm not sure how one could do the math.
Yard sales are probably the best places to pick up useful items that other people have in overabundance, Such sales are especially good in areas with a transient population, such as university towns. Or affluent towns! I was once at a conference in Clarement CA and--on a walk--passed millions of yard sales with incredibly tempting items--all off limits to a plane traveler, alas. I myself don't go to more than two or three yard sales a year. The best of the best: church sales and similar. Only two that I go to. Thrift stores are my choice, even though they are more expensive than most yard sales.
OK, so over the years I have stocked up on
--cotton and linen dish towels
--cooking utensils (ladles, spatulas, whisks, etc)
--mid level pots and pans
--plastic storage, preferably Tupperware (but beware, often T-ware is up-priced)
--mediocre linens*** (good linens are preferable, of course, but mediocre ones are fine for students, campers, etc)
--mismatched dishes and cups (I only buy Corelle)
--mismatched glasses (I have acquired a set of my favorite Picardie one at a time)
--"" silverware (when I can find most of a set)
--coffee makers (thanks donors for the great French presses)
Hmmmm. This is a rather pathetic list. Most of these items cost only a few dollars. Some--like the linens--might evoke the squeam factor*** or EWWWW disgusting factor****.
Some require that you relinquish certain conventions, like having matching dinnerware. (I do have matching Corelle at this point, btw).
If you look at the list, you might say: Oh, all this stuff would only cost a few hundred dollars, max. I guess that's true, but I still think that these little savings add up and compound in ways that my math cannot accommodate.
All I know is that I set up "house" for my children when they went to summer camp (my daughter was embarrassed by her linens--the other girls all had matching themed sets. I found one at the thrift for her next summer). I set up house when they went to college and when they went abroad for studies.
When my son got an apartment in New Orleans, I could set up his kitchen and bath instantaneously. This saved both money and time. When something breaks, I usually have a back up: that saves money and time also.
I am convinced that this saved money compounds somehow. Does anyone know how one could figure that out?? My fave glasses....