@joannaholman some (imo) fascinating details about how these stores work:
They sometimes buy just bulk stuff at auction like an entire written off trailer or trailers full of stuff where one of the parties just stiffed them.
They also get their stuff from storage units and stuff, you can buy an entire defaulted storage unit for like $500-1000 (in the US) or less, it's a bit of a gamble but like it can work out.
They also buy things from insurance companies who've written off the goods and are just selling them for whatever they can get for them, and there aren't often many bidders so you can get things really cheap.
Also they bid on returns from Amazon and eBay sellers, these often wind up at these shops. It can be a bit of a gamble, but a lot of returns are just "I didn't want it" or "I got the wrong item."
These places can often end up buying anything from a pallet full of lawn chairs, to a batch of extremely obsolete knockoff smartphones that never sold. Some wholesalers will sell the pallets direct, even.
@joannaholman I knew someone who did just that. You need a bullshit radar that's in tip-top shape. Because sometimes you might see a shiny storage unit that looks like it's full of rare collectables but then it turns out they're all Chinese knock-off figurines.
eBay and Amazon returns are always rolling the dice, sometimes you get a pallet and like 80% of the stuff works, sometimes you get a pallet with maybe 1-2 working items.