More to the point: Why is there NEVER a garage being sold at 'garage sales'? Or a yard at 'yard sales'? The only sign that makes sense is 'rummage sale'.
For those not familiar with the reference, Grammar Girl is Mignon Fogarty's awesome blog on the more idiosyncratic parts of the English language. I highly recommend it: It's a fun, interestin' read on language 'n learnin' 'n lingual luminations. <--This sentence would get me in trouble over there.
I used to do this [correct signs] in the halls of the high school where I worked . . . come on, students and teachers - this IS supposed to be a place of LEARNING (isn't it?) . . .