The Glen selling 2nd hand items

The store at the Glen sold me a 2nd hand Netgear Router which a previous customer returned to them as it was faulty. On returning it I was issued a gift card as they had no cash to refund me. I then purchased a router at Southgate which also did not work and now they will not refund my money even though I paid cash for the item. The sales staff advised the manager of the reasons for the gift card but his attitude is that they are only “salesmen” and he is the manager so what he says goes. This manager is selling damaged goods at this store as 2 weeks ago I phoned around for stock on a DELL laptop and the salesmen at The Glen advised that although they showed 2 in stock on the system one was a demo and the other model was used and damaged stock so I should not purchase the item.