Worse experience ever

FirstShop retailer was selling a CCTV DVR and on the advice they have given me that it was compatible with my cameras I purchased the item but on arrival and connecting I realized that my cameras was in fact not compatible with their DVR and asked them for a refund but they refuse as their reason is I opened the product and used it so they cannot take the item back for a refund. Will never purchase from them again and I reported them to the relevant authorities.