I've gotten many, many cool things, but none at garage sales. I know people find stuff all the time, but I have never found one interesting thing at a garage sale. eBay is a whole different story, and I've found some amazing outre things there, like my Godzilla bubble machine, and a weird magazine called Echo, that you both read and play (five of its pages are 45rpm-sized records). It has an interview with Gypsy Rose Lee talking about the musical Gypsy, and then a record of Jule Styne playing songs from the show at his house (including, I think, some rare stuff). It has a story about Nichols and May, and then a record of them talking at breakfast. James Thurber contributes a piece about harmoica player Larry Adler and then there's a record him Adler playing. On on it goes. I think it died after this one issue.
Of course, I've found some great Li'l Abner stuff, including the one of the best original panels of Capp art I've ever seen.