Mercari is basically a peer-to-peer marketplace similar to a mix of eBay and Facebook Marketplace, and the process is fairly simple once you understand the workflow. As a seller, you create an account, list items with photos and descriptions, set your price, and then either offer free shipping or have the buyer pay shipping.
Once an item sells, Mercari provides a shipping label if you use their shipping system, and you are generally expected to ship within 3 business days. After the buyer receives the item and rates the transaction, the funds are released to your balance, or Mercari auto-completes the transaction after a few days if the buyer does not leave feedback.
Mercari is very beginner-friendly and works especially well for smaller items, collectibles, clothing, electronics, beauty products, and second-hand goods, but compared to Amazon it is much more casual and individual-seller focused.
One important thing to know is that buyer communication, ratings, and accurate descriptions matter a lot there, because the platform relies heavily on trust and seller reputation.