This is very informal French. It's written without punctuation, so the first step is to add it back:
C'est à vendre ou c'est pas à vendre ? Faut pas se foutre du monde !
The first sentence is asking whether ”it“ (an object which should be clear from the context) is for sale. The question structure with the subject before the verb is the informal question structure of French, where only the intonation marks that the sentence is a question rather than a statement. The repetition “is it … or is it not …?” is generally emphatic, and specifically connotes insistence: the person who is asking wants an answer now.
The second sentence can roughly be translated semi-literally as “you must not bullshit people”, and really means “stop bullshitting people” or “stop bullshitting me”. “Se foutre de” can mean many things, centered around not caring about something or someone; it's always highly informal and quite rude. In “se foutre du monde”, it means doing something outrageous; despite the presence of monde (world) the outrage might only be felt by a single person (it's an idiom).
Thus the potential customer is casting doubt as to the seller's willingness to sell, and accusing the seller of doing something wrong — at least of wasting the potential customer's time. All of this very impolitely.