I would say that they're not entitled to a refund on the RAM if the retailer is able to offer a repair or like for like replacement. You can ask fora refund, they may oblige. You may also be able to get one if you can prove there's an inherent design fault in the RAM, I think that may buy you some bargaining room with the retailer, but I don't think it's a guarantee. In this situation, if I'd simply changed my mind I'd accept the replacement and sell it on elsewhere. You'll lose some money, but it was your mistake to purchase the product without researching it first.