Literally, the "Goodwill" store: About 15 years ago, A Fairchild Channel F. Wanted $20 for it which was high but only one I've ever found.
At other 'thrift' stores around the same time: A box of ColecoVision boxed cartridges with manual and accessories: $40 and had about 50 games, 15 of which were quite rare. Otherwise just numerous Atari 2600, 5200, 7800 cartridges, systems and accessories, nothing spectacular.
Best find ever: At a pawn shop, who wanted $5 a cartridge for years for a tote with nothing but Combat, Pac-Man and other common 2600 titles most missing labels, suddenly got a new 2600 deck pawned with the plastic storage case. Under the deck was a whole bunch of new carts. My eyes popped open and jaw dropped....
Snagged about 10 of them at the normally high price during a 20% off sale. In the batch: Atari 2600 Springer.... Atari 2600 Q*Bert Cubes.... Intellivision Pac-Man...... Intellivision Test/Demo cartridge, and .................. Atari 2600 Chase The Chuckwagon! <SCORE!>

Never found a better single day score of any home system parts before or since.