the first thing you do when

Make sure to meet the seller in person and verify their identity. It is preferable to meet them in a public place with a friend present.

Make sure to inspect the product and have it checked by experts.

Document the sale/purchase process, including full details of the product.

Do not transfer any money until you have verified the following: (the seller's identity, the documentation of the sale, and receipt of the product).

Make sure to obtain a signed receipt from the seller.