The whole idea of vetting makes me uncomfortable. For what purpose would the material be vetted? To keep people from reading untrue/objectionable ideas? And why is the bookseller's idea of what is true/untrue, acceptable/objectionable, something I as a customer should accept? Isn't the potential logical extension of that something like "don't say gay" or book banning? I don't think that's slippery-slope reasoning, necessarily.
Who gets to decide what I as an individual can read? I can decide what to spend (or not spend) my money (and time) on; that's not the job of a corporate or individual bookseller, and I'm loath to relinquish that power. Of course, I know that such vetting already exists in places like Christian bookstores, as an example; I exercise my choice by declining to shop there because they've made choices about what I should/not be able to read via the selections they offer.
I might well, in fact, buy the same books that a Christian bookstore offers, but I'll do so from more "open" or broad-based offerings at other retailers. I consciously and specifically patronize those stores who leave the choice of what I read up to me, rather than financing the places who believe it's their mission/job/business model to try to take those choices away from me.
Finally, it sort of seems like the entire idea of a bookstore refusing to sell "slop" (however that is defined) isn't really a meaningful step toward a more educated, informed citizenry in today's world. The current and growing availability of misinformation online (particularly via very effectively crafted and targeted social media), coupled with spotty critical reading and critical thinking skills among Americans as a whole, seem to me to take a lot of the wind out of the sails of the need for carefully curated bookstore offerings, and of books in general. If the past few years have taught us anything, I'd think it would be that an insistence on objective truth doesn't carry the weight with the masses that it used to (and still does for many of us).