The odds of finding Bukowski books in a thrift store would have been better in the past. These days, many (most?) thrift stores have staff pickers (or outside "experts") who go through the incoming books and pull out anything of interest. Either these are sold to the expert at below retail value, or they are listed on-line. The good books in these shops never hit the shelves.
In my town, there's one guy who has gotten himself in with every thrift store in town, and he skims the cream on all the books donated to thrift stores, as well as a bunch of annual rummage sales and library sales. Book scouts speak of him with disdain. I've never seen the guy, but I see the effects of his cherry-picking all the time. Once in a while, he screws up and lets a rare book through, but not often. I wonder what sort of premium price he pays for the good stuff? I bet it's low, like $1.50 instead of the usual $1.00 they would charge the public for the book if they put it out on the shelves.