I think there are a few different factors at play with this subject.
I have read somewhere that Goodwill management and the main office has compiled a list of desirable items and distributed this list to all the stores to be on the lookout for these types of items. If anything on the list does show up the store is supposed to forward it to the main office so they can post it for sale on the auction site.
Now I believe some good items do slip by from time to time but some stores have employees that will keep good stuff for themselves or set it to the side and call a friend who might be interested in it.
For the rest of the good stuff that does slip by,which probably isn't much you need to be in the right place at the right time to score it.
There are many people with lots of time on their hands that frequent the stores daily looking for deals.
Not long ago I did get very lucky with some 90's speakers. I ran across these speakers and had never heard of them but they got my interest because of the beautiful gloss piano black finish,then I realized how heavy they were. I stood next to them and googled them,after a little research I found out they were NHT 2.3a speakers from 1993 and sold for $1200 new. I thought these must not work and have some bad drivers. For a total of $16 I figured it was worth a gamble,bought them,took them home and everything works perfect on them,they have no big scratches,very nice looking,grills are very nice. They sound fantastic! I got lucky and was in the right place at the right time.