What is the ruling on buying from a student of mine (who works as a seller) if I fear he might give me a special price out of need, even though he affirmed he would not?

First: If the teacher is a government employee and has authority over the trainees, then their gifts to him are considered bribery (rishwah) and unlawful misappropriation (ghulool), which are forbidden. However, if the teaching is not formal or the teacher does not have authority, there is no harm in accepting gifts.

Second: There is no harm for the teacher to buy from his student at the usual price, without apparent favoritism.