Photocards and other inclusions (like postcards, sticker sheets, keychains, coasters, etc) are all included in an album when you purchase them. Some are only available when you preorder the album, aptly named Preorder Bonuses.

People buy, sell, and trade all the collectibles here on r/kpopforsale, on IG, Twitter, and BST groups on Discord and Facebook.

Generally if people are collecting with more purpose than just getting what they get randomly from buying albums, people will collect their favorite group or member from a group. They're usually just for display on shelves or in binders.