In order to increase the chances of every sale, ideally sellers list their products through various marketplaces. Each marketplace have their own requirements and it is a time consuming activity to upload photos, writing product descriptions and so on. Besides some sellers having their own website may require someone to manage the entire catalog.
You can earn by uploading the products in different marketplaces, manage their daily orders, etc. This may be one-time or, repetitive activity and you can charge accordingly. Learning curve will be longer, depending on the number of platforms you choose but the overall process will be similar.
While some sellers have limited products, other sellers have tens to hundreds of products; you may charge extra depending on the time taken to manage number of products (keep in mind, uploading each product requires precision and it is time consuming). For example, charge $ 2.00 for every additional product beyond first ten or twenty products. It is really up to you and the customer to decide on the pricing.
Here is a small list of popular B2C & B2B websites, through which the products can be listed and sold:
As each vendor have their own process for product management (and they may update the process time-to-time), it is recommended to use their own tutorials or documentation.
Note: We will update this list as we discover more; please add other websites in the comments and we will include them in this list.