When purchasing wool noils, the most critical factors are fiber length, micron count (fineness), and Vegetable Matter (VM) content. High-quality noils should have a consistent fiber length (typically 15-30mm) to ensure even blending in woolen spinning. The VM content should ideally be below 0.5% to 1.5% for premium grades; higher VM requires additional carbonization, which increases processing costs. Additionally, verify the color/brightness, as yellowish noils may limit dyeing options for pastel shades.