Learn Farsi: بعضی از مشتری‌ها خواستار تخفیف هستند. - Some of the customers are seeking a discount.

It perfectly demonstrates the pattern of past stem + ـار. The past stem of خواستن (to want) is خواست. Adding the suffix ـار creates a manifestation noun or adjective indicating someone who embodies that action. Here, it translates to 'seeker' or 'requester'.

Is there an Ezafe connection between خواستار and تخفیف?

Yes. Because خواستار functions as a noun here, it needs the unseen Ezafe vowel to connect it to the object being requested. You pronounce it as khastar-e takhfif, which literally translates to 'seekers of a discount'.

Could I just say تخفیف می‌خواهند instead of خواستار تخفیف هستند?

Yes, you absolutely could, and it carries the same core meaning. However, using the derived noun خواستار with a 'to be' verb is a great way to sound slightly more formal, professional, or journalistic. It elevates the tone from 'they want' to 'they are requesting'.

Does بعضی از always require a plural noun like مشتری‌ها?

Yes, when بعضی از (some of) refers to countable things or people, the noun that follows must be plural. Also notice that the main verb هستند is plural to agree with this subject.