Buy now, pay later sales
These are quite different from layby sales. You get the goods as soon as you make the first payment. Since you have legally bought the goods, you can’t cancel the sale once you have made that first payment. Payments are not flexible like laybys, instead they will be locked in, weekly or fortnightly, and if you miss a payment, penalties will probably apply.
You are within your rights to return faulty goods and get a refund of your payments to date, or you could ask for a replacement or repair.