If you are purchasing a resale through a broker, it is anticipated that you may make an "offer". Things to be negotiated include the price, the period of time before closing, who will pay closing costs, financing(owner vs. cash), etc.
Everything is acceptable to negotiation. Even if you offered "full price", you would still be negotiating terms.
Frequently, if you are buying from a developer, you consider it unnegotiable, because you are paying the asking price. But you may still negotiate minor items like closing and financing.
FWIW
Michael