I agree with Grannie Annie. Actually, the traditional garage sale price for anything in good shape is 10 cents on the dollar of current retail. This has been the case for generations (remember "Second Hand Rose"?). The concept is that people are really eager to get rid of their stuff and don't want the bother of selling some other way (newspaper ads, eBay, Craigslist, etc. - or how about right here on the QB?!). Most methods of selling provide category sorting, or even, in the case of things found via computers, very specific search responses. When you go to a garage sale you have invested gas and a lot of your time gambling that there might possibly be something you'd want at a big saving.
That said, if I found your fabric at the price you were asking I would have snapped it up and been quite delighted. That's because I'm a quilter and a fabric fiend. The chances are pretty good that people who spotted the fabric at your garage sale are neither. They were just looking at it as having some possible purpose maybe. They're not invested in the idea of getting good fabric in the same way that we are and may not know what it's worth, and so they want it dirt cheap.
I had a few garage sales but got discouraged because - although the vast majority of people are honest - every time there was at least one rotten apple. One time I was having a sale with a neighbor and someone pulled the price sticker off a cheap item and put it on a much more expensive item of hers. They came up when I was very busy and distracted (possibly by someone working with them, interrupting me with a question), and I messed up and sold the item before I realized what happened. That was my last garage sale.
It can be quite dangerous to confront strangers who have already demonstrated for you that they do not respect social norms. They just might be crazy enough to pull a gun. I would have done as you did and chalked it up to experience.