An important skein of homespun wisdom collected from my years at the shop is never give the volunteers a hard time.
Nobody's getting paid at a library sale, nobody's getting paid at a thrift store, and they're endlessly hassled by legions of scroungers and bottom feeders. It's gotten bad enough at one of my regular stops they've posted a big NO HAGGLING sign at the register.
I always go out of my way to be amenable. As I'm nosing around I'll straighten shelves, at a couple of my stops I'll haul books to the Goodwill for them (protip: around here, *everything* eventually ends up at the Goodwill), I'll help them sort the saleable stuff from the total junk in the back room, etc etc.
And while advantage seeking isn't the motivation, human beings being human it does pay off.
When you buy a ton of stuff & you're not a jerk about it, there are perks.
Today's bit of slack came when the entire register system crashed at the outlet, so no credit or debit cards. I had to run and grab some cash from an ATM- by the time I got back there was a ten deep line moving at a snail's pace and I was supposed to have the shop open in five minutes.
A brief chat with the manager & I was on my way.
"Remember the count and we'll ring it up next time."
Chalk up another win for common decency!