Never at a garage sale but have had great luck at local flea markets/auctions.
Got this lot for like $75 a few years back:
1968 Topps Nolan Ryan RC raw - ended up grading a PSA 4.
1968 Topps Mickey Mantle raw - ended up grading a PSA 6 MC
1969 Topps Reggie Jackson RC raw - ended up grading a PSA 2
1979 Topps Ozzie Smith RC PSA 8.
Got this lot for less than $100 a number of years ago:
2010 Bowman Chrome Julio Teheran auto
A bunch of 2011 Bowman Chrome Danny Brewer autos/plates that were all /5 or less including a couple superfractors. Even though he was out of baseball at the time I bought, I sold the auto super for the price I paid for the lot and everything else was pure profit.
Plus some other bowman chrome autos