Is it possible to use Moralis to display an NFT collection
The Moralis NFT API will only show data for NFTs that are on-chain - you can check the contract address and tokenId(s) on the contractโs chain explorer e.g. Etherscan. If they are only lazyminted, then they wonโt show - on OpenSea at least, one way to get them on-chain is to transfer the NFTs to another wallet.
offer the possibility to the users to buy theses NFTs directly from our app without having to go to Opensea or Rarible ?
If this is possible, it would need to be done through interacting with their contracts or API. You can ask in their communities - OpenSea and Rarible.