A series of glass plate negatives from famed nature photographer Ansel Adams were discovered from a random box of "junk" purchased from a garage sale. I think that these kinds of finds are definitely a fun part of garage sales: you never know what you'll discover, and more often than not they are worth a lot of money.
Yes, garage sales are a place to get valuable goods. Just look at this example here - a man purchased two boxes of items for $45 dollars that was worth $200 million. Garage sales are a great places to find valuable goods for people at great prices. Garage sales give people a large variety.
Garage sales can be a treasure trove, but they can also just be a lot of everyday ordinary items. It's often newsworthy when a garage sale bargain turns out to be a valuable item. Pottery and ceramics, older photographs, documents and paintings, records and furniture finds turned out to be worth many times more than the sale price.
Some people have found some amazing finds at yard sales, and there are a few people who have managed to make careers out of it but most people who sell things at yard sales are trying to get rid of junk. People generally know if they have something that could be considered valuable.