Neutral term for a person who buys something, often used in commercial or legal contexts.
Purchasers of this product will receive a bonus.
購入者 is a more formal, written term for 'purchaser' used in contracts and documents, while 買い手 is the everyday business word.
購入者 is a general term for anyone who buys something, while バイヤー specifically means a professional buyer or purchasing agent in a business setting.
Compound of 購入 (purchase) + 者 (person).