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Here on Hive they are stored within their dapp. This art nft is stored at nftshowroom.com . Splinterlands nft cards are on their site. Same with RisingStar. I say on their site, but it's your hive account on their site and each of those have markets built in.

you can also trade. I recently traded cards with another Rising Star player to get a card i needed for the game. :)

You can make NFT's on other blockchains, but most use ethereum and the gas fees are too high for me. I like it here better anyway.

Oh, and I almost forgot. Most of the nft's on hive can be seen and bought here too
https://tribaldex.com/openhive