I guess this is another case of the seller, did not test the equipment in the presence of the buyer. But den storing in the store for 2 months and not testing it when you reach home is weird. There is a risk if you buy 2nd hand equipments, It is best that you check and test before buying..
And Hydor product might be a bit wonky in the first place.. =X
If a refund is offered, why should you refuse the refund?