# Charge a fee for mystery card purchases · Decentralized marketplace with FT4 · Learn Chromia > The minting lesson introduced operation buy_mystery_card, which mints a random card for the caller. To turn it into a real purchase, you need to define a… Source: https://chromia.com/learn/marketplace-course/module-assets/buy-mystery-card/ Decentralized marketplace with FT4Lesson 11 of 15 1. [01Decentralized marketplace with FT4](https://chromia.com/learn/marketplace-course/introduction/) 2. [02Set up your project](https://chromia.com/learn/marketplace-course/setup/) Module 1: Registering accounts and assets 1. [03Overview](https://chromia.com/learn/marketplace-course/module-ft4/) 2. [04Register payment token](https://chromia.com/learn/marketplace-course/module-ft4/register-token/) 3. [05Create user accounts](https://chromia.com/learn/marketplace-course/module-ft4/register-account/) Module 2: Model NFTs in Rell 1. [06Overview](https://chromia.com/learn/marketplace-course/module-nft/) 2. [07Define the NFT model](https://chromia.com/learn/marketplace-course/module-nft/nft/) 3. [08Mint NFTs](https://chromia.com/learn/marketplace-course/module-nft/mint-nfts/) 4. [09Make card attributes random](https://chromia.com/learn/marketplace-course/module-nft/randomness/) Module 3: Create the marketplace 1. [10Overview](https://chromia.com/learn/marketplace-course/module-assets/) 2. [11Charge a fee for mystery card purchases](https://chromia.com/learn/marketplace-course/module-assets/buy-mystery-card/) 3. [12Put a card up for sale on the marketplace](https://chromia.com/learn/marketplace-course/module-assets/list-card/) 4. [13Buy a card on the marketplace](https://chromia.com/learn/marketplace-course/module-assets/buy-listed-card/) 5. [14Test our marketplace](https://chromia.com/learn/marketplace-course/module-assets/test-marketplace/) 6. [15Test using Chromia CLI](https://chromia.com/learn/marketplace-course/module-assets/test-cli/) [Learn](https://chromia.com/learn/)[Decentralized marketplace with FT4](https://chromia.com/learn/marketplace-course/) 11 / 15Module 3: Create the marketplaceAdvanced # Charge a fee for mystery card purchases The minting lesson introduced `operation buy_mystery_card`, which mints a random card for the caller. To turn it into a real purchase, you need to define a price and charge the user that amount each time the operation runs. Each payment moves from the user to a dedicated dapp account that collects all card-sale revenue. That economy account does not exist yet, so you will create it with FT4. Add a new entity, `dapp_account`, to hold it. This lets your Rell code reach the account directly. First, define a structure in Rell that references the account. src/rell\_marketplace/module.rell ``` struct module_args { dapp_account_signer: pubkey; } ``` Next, create the account inside the `dapp_meta` object you declared earlier. src/rell\_marketplace/module.rell ``` object dapp_meta { asset = Unsafe.register_asset("Collector Card", "CRD", 6, chain_context.blockchain_rid, "https://url-to-asset-icon"); account = create_account_with_auth(single_sig_auth_descriptor(chain_context.args.dapp_account_signer, set(["A", "T"]))); } ``` This code calls `create_account_with_auth` to create the account and passes it a single-signature auth descriptor with the following values: * The dapp account pubkey, read from the moduleArgs settings * The flags "A", which allows managing accounts, and "T", which allows token transfers from this account Now generate a new keypair to act as the dapp's "account owner": shell ```nolines chr keygen --file .chromia/dapp-account ``` Open `chromia.yaml`, add a module argument for the economy account, and set its value to the `pubkey`. chromia.yml ``` blockchains: rell_marketplace: module: main moduleArgs: lib.ft4.core.accounts: rate_limit: max_points: 10 recovery_time: 5000 points_at_account_creation: 2 lib.ft4.core.admin: admin_pubkey: x"" # Replace with previously generated public key here. rell_marketplace: dapp_account_signer: x"" # Replace with previously generated public key here. compile: rellVersion: 0.14.9 database: schema: schema_rell_marketplace test: modules: - rell_marketplace_test moduleArgs: rell_marketplace: dapp_account_signer: x"" # Replace with previously generated public key here. lib.ft4.core.admin: admin_pubkey: x"" # Replace with previously generated public key here. libs: ft4: registry: https://gitlab.com/chromaway/ft4-lib.git path: rell/src/lib/ft4 tagOrBranch: v1.2.0r rid: x"E96915FD7C9A5DD4E183D86F4A5B4BB5083A7CC1EDEB1465EDFFC926E67C93F6" insecure: false iccf: registry: https://gitlab.com/chromaway/core/directory-chain path: src/lib/iccf tagOrBranch: 1.87.0 rid: x"9C359787B75927733034EA1CEE74EEC8829D2907E4FC94790B5E9ABE4396575D" insecure: false ``` You now have an account that can receive charges. When a user buys a card, for example, the amount moves from their account to this one. To apply this, add a transfer to the mystery card purchase. The transfer takes these parameters: * From account * To account * Asset * Amount With the transfer in place, `buy_mystery_card` looks like this: src/rell\_marketplace/marketplace.rell ``` operation buy_mystery_card() { val account = auth.authenticate(); Unsafe.transfer(account, dapp_meta.account, dapp_meta.asset, 100); mint_card(account); } ``` The card price is fixed at 100 CRD. Each call moves 100 CRD from the authenticated account to your `dapp_account`. Written for this site from Chromia’s public course. Code samples are the course’s own. [← Previous**Module 3: Create the marketplace**](https://chromia.com/learn/marketplace-course/module-assets/)[Next →**Put a card up for sale on the marketplace**](https://chromia.com/learn/marketplace-course/module-assets/list-card/)