If you're told that a price is negotiable, that means you can talk it over until you reach an agreement. So don't start with your highest offer.
Negotiable can also mean that a road or path can be used. Since the avalanche, you've found that many of the local roads are no longer negotiable. If you can pass on a possession to someone else, making them the owner, then it's said to be negotiable. The "t" in negotiable is pronounced "sh."