In my experience garage sales were ok, but yes there was always that fear that some item could contain wicked spirits or demons. So a lot of JWs cast a wary eye at anything bought at thrift stores or garage sales. If it is a church garage sale, then the answer is absolutely definitely not.
If anything is church related, its pretty much a no-no, even if the cause is seen to be good and noble by persons who are not JWs. To JWs, all other religions are seen as Babylon the Great, controlled by Satan the Devil. So any efforts carried out by these agencies would be indirectly supporting an entity controlled by Satan in the mind of the JW.
Axelspeed