This isn't necessarily true. A lot of people will sell off their iDevices without being aware the iCloud lock is a thing at all, and unless the device is formatted before being sold to the second hand store (which is less likely than you think), you usually won't know it has an iCloud lock until the store formats the device themselves, and at that point they usually don't bother with in-depth checking at all. Then you have places like Goodwill and other thrift stores where people will just toss whatever at them and they don't even get turned on or tested at all. There's also eBay and other auction sites as well that will sell off iOS devices that aren't tested beyond boot as well.
The only time I'd say this statement is true is if the second hand store lists themselves as "Apple certified", or have a strict policy and don't accept devices until they're formatted and can verify no iCloud lock...which most places don't.