Sometimes we refer to luxury used in this way as a 'noun modifier'. It helps to view 'luxury goods' as all one word - much as we would things like 'fire extinguisher', 'express train', 'general waste' etc. The main way we can think of them differing is in our expectations. Luxury goods are - by definition - fancy, beautiful and so on. On the other hand, luxurious goods are goods which happen to be luxurious. For example, I might go to a second-hand store to buy things that other people don't want. These goods are definitely not luxury - someone has thrown them away. But they might be luxurious, because they are made from fine materials.