Side Note - Well f-me… Gumtree is owned by eBay…
Onto the topic at hand…
The issue to some degree would come down to the local state laws around pawn brokers, but in general it is the pawn broker’s responsibility to ensure that the goods are not stolen. (Har har; I don’t think it’s a hidden reality that a lot of goods sold by pawn brokers are likely stolen…)
In QLD for example pawn brokers are meant to record details like serial numbers, and both the seller and buyer’s details, so in the event that a device is found to be stolen, the goods can potentially be reclaimed for the original owner, and the seller pursued for their part in the transaction (ie selling stolen goods).
The reality is - iPhones can be iCloud locked without being stolen property. As such, until evidence emerges - they aren’t doing anything illegal.
Are Apple supposed to keep track of stolen iPhones in AU? I think they have to in the US? Perhaps your uncle should contact Apple, but this time ask them to check if the device is listed as stolen (explaining his situation of course!). If it IS, then go back to Cashies with that information… (Or the police…) Pretty sure a refund would be forthcoming then…
(I would still try my above option first - Even tell him to “ask” (loudly!) “Does this mean the iPhone was stolen!?”)
Ultimately - sadly, it’s another reason I’m doubting the idea of buying a second hand iPhone again… If there wasn’t enough issues over the likelihood of getting something that’s “3rd party refurb from O/seas”, there’s also the iCloud lock to consider… Apple must love it.