I go to a lot of garage sales (over 20 years since my daughter was a baby and needed a constant supply of clothes, toys and books). I NEVER expect to find something -- it's always the thrill of the chase! (although a neighbour once disparagingly described it as "kissing a lot of frogs before you find a prince"!) I have bought a 201K Singer for $11.50 on ebay -- 2 weeks before Christmas and obviously no-one else could be bothered at that time. So I guess my message is to enjoy the hunt and where it takes you -- you might end up with some new pot plants, a nice vase, a book you want to read-- and you might find a sewing machine that the owners didn't even bother to list.
Happy hunting!