When I'm out scrounging books I enjoy it when other scroungers wander in, see me rooting through the stacks and walk right back out. It's a minor triumph, but these days I take what I can get.
It's something I never do- I have the old school attitude that I can find good stuff anywhere, however junky the books, however picked over the buy. Seeing another scrounger on the beat is a challenge and turning up a really good book they missed is a definite high.
And everyone misses stuff. The scanner borg have their weaknesses, real people have their own areas of expertise & prejudice. The other day I found a good book the boss couldn't be bothered with on our dollar sale cart. I picked it up and sold it this morning for ten bucks.
I always assume I'm going to find good books.
Most of the time I do, and anyway there's no percentage in assuming someone else already got all the good stuff. To paraphrase that immortal philosopher Woody Allen, "90% of finding good books is just showing up".