Even cash on collection can be returned when the item is sold online. Although it's far more difficult. So I would only recommend buying GPUs from OcUK etc where you hopefully get a good RMA situation.
For example seller lists a Mined GPU on said site for £1500 as good condition. Sells it for cash on collection. Buyer collects and pays cash but gets home uses it for a day/week and the card stops working properly.
The buyer would be still entitled to a refund. The only thing with cash on collection is that the seller has the money in cash. The buyer can still proceed to get their money back via confronting the seller/ via courts if the seller does not cooperate. But this also depends on what site and what terms were agreed.
For high value items like that I am sure the buyer will most likely pursue the money through small claims if they are sold a dud, assuming they know their rights and can fill in a few forms online.