Learn Farsi: پندار آنها درباره‌ی خریدار غلط است.

It is formed by taking the past stem of the verb خریدن (to buy), which is خرید, and adding the suffix -ار (-âr). While this suffix often creates nouns showing the manifestation of an action (like رفتار for behavior), it can also create agent nouns like this one, meaning the person doing the buying.

Is پندار (thought) also formed using the past stem + -ار suffix?

No, it just looks similar! The word پندار is actually the present stem of the verb پنداشتن (to suppose/think) functioning directly as a noun. The past stem of that verb is پنداشت, so it doesn't follow the exact same past-stem rule as خریدار.

Why does درباره (about) have a ی at the end here?

The word درباره ends in a silent 'e' (ه). Because it acts as a preposition relating directly to the next noun (خریدار), it requires an Ezafe to link them. For words ending in a silent 'e', the Ezafe is written with a ی and pronounced as '-ye', making it درباره‌ی (dar bâre-ye).

What is the difference between پندار and the more common word for thought, فکر?

فکر (fekr) is the standard, everyday word for a thought or idea. پندار (pendâr) is a more formal or literary word of Persian origin. It often implies an assumption, notion, or even an illusion (something supposed rather than proven). In this context, it emphasizes that their assumption about the buyer is a false notion.