Garage Sales. On weekends, it is not uncommon to find families selling no longer needed household items in their driveway, garage, or yard. If you see a driveway full of stuff on a Saturday, it's likely a garage sale. Check it out; one person's trash may just be your treasure. Bargaining is expected.
Flea Markets. Flea markets (called "swap meets" in Western states) have dozens if not hundreds of vendors selling all kinds of usually inexpensive merchandise. Some flea markets are highly specialized and aimed at collectors of a particular sort; others just sell all types of items.
Auctions. Americans did not invent the auction but may well have perfected it. The fast paced, sing-song cadence of a country auctioneer, selling anything from farm animals to estate furniture, is something not to be missed, even if you have no intention of buying.