In some circumstances customers don't have a legitimate right to complain about goods.
Customers in Northern Ireland can't complain about goods if the purchase was more than six years ago.
Neither can they complain that goods aren't of satisfactory quality if you pointed out the defect before the sale. Similarly, if the customer examined the goods before buying them and a defect was obvious, they can't later reject them as unsatisfactory.
Customers can't reject goods as being unfit for their purpose if you made it clear when you sold them that you were unsure whether they would be suitable.