Every time you collect a new item, whether it’s normal, a material, a piece of equipment, a key item or even a recipe, it is recorded in the game’s encyclopedia function. You can access this by going to Misc, then Info, where you can see how many items you have obtained during the course of the game. To earn this trophy/achievement, you will need to have registered 500 different items. It doesn’t matter if you sell or get rid of items, as the game keeps track via the in-game encyclopedia feature. You should get this naturally, especially if you’re going for another trophy/achievement, like Dedicated Follower of Fashion.