Faulty MacBook Pro Charger

I’ve put in a claim to replace a faulty MacBook Pro power brick. Back market have told me the vendor does not have a suitable accessory and therefore cannot send a replacement. BackMarket have now offered me £25 to buy one, yet their prices for one on their website are starting at around £40 for my model. Surely I should not have to pay out of my pocket to replace a faulty item from them?