> ## Documentation Index
> Fetch the complete documentation index at: https://sequence-0fb8d9e6-api_docs.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Introduction

Secondary Sales / Peer to Peer Trading is facilitated by our [Marketplace API](/api-references/marketplace/overview).

Users will be able to purchase and sell ERC721/1155s between one another via the marketplace SDK in Unity; paying in cryptocurrency (with currency abstraction enabled by default) or via credit card where available.

## Understanding the Basics

Before diving into the details behind the marketplace, let's first establish an understanding of some basic concepts.

### Orders

There are two basic types of Orders: listings and offers.

`Listings` indicate an intent to sell a given amount of a collectible (where a collectible is the combination of a Chain, contract address, and token id) at a certain price per collectible in a given Currency. Once created, listings can be queried and bought by another user.

`Offers` indicate an intent to purchase a given amount of a collectible at a certain price per collectible in a given Currency. Once created, offers can be queried and sold by another user.

In order to create, fill, or cancel an Order, you must use the appropriate Generate\_Method\_Transaction call based on which method you are using. This will give you a `Step[]` that can be submitted as a transaction in order to perform the desired action. More on this in the [Filling Orders section](/sdk/unity/monetization/secondary-sales/how-it-works/filling-orders) of the docs.

### Currencies

A Currency is an ERC20 token or native token that has been whitelisted for a given project and Chain for use in the marketplace. You can add a new currency to the whitelist for you project by adding it as a custom currency when creating a white-label marketplace in the Sequence Builder. Please see [this guide](http://localhost:5173/solutions/payments/marketplace/hosted#add-a-custom-currency). All orders must be priced in whitelisted currencies or they will be ignored by the API.

### Collections

A Collection is a group of Collectibles, identified by an ERC721 or ERC1155 contract address.

### Collectibles

A Collectible is a specific instance of a Collection, identified by the ERC721/ERC1155 contract address and the token id.