Buyers must prioritize the chemical composition and mechanical properties of the alloy. For instance, the 6000 series (6061, 6063) is popular for extrusion due to its excellent strength-to-weight ratio. Ensure the supplier provides a Mill Test Certificate (MTC) that details the levels of Silicon, Magnesium, and Iron. Additionally, specify the billet diameter (e.g., 3 to 12 inches) and length, as well as the homogenization process used, which is critical for ensuring uniform grain structure and preventing defects during downstream extrusion or forging.