In the sentence

You b*tch when I buy you things.

is the word "b*tch" being used as a noun or is it a verb with a definition equivalent to the word complain?

I think b*tch would be a verb, and the noun would be things.b*tch would be a verb because it is an action if you are using it instead of complain

The noun is 'you', and is also the subject.
The verb is 'b**ch', as it saying what's she's doing.

'when I buy you things' is a prepositional phrase.
'when' is a preposition.

B*tch is being used as a verb in this case, because it is suggesting something that is 'done'. It is an equivalent to 'complain', but would be classed as 'slang' and therefore, in my experience as a teacher, would not be an acceptable term in student's written work.
The noun is 'things'.