Negotiation
I highly recommend this 10 episode series about negotiation. It introduces how to open negotiation, how to do some negotiation, and what to watch out for from the other party.
My favorite episode is 10: salary negotiation. How to ask for what you think you're worth, and an idea about the gender pay gap.
The Most Useful Thing I Learned in this Series
Just ask! As the buyer, you have every right to ask if the price is negotiable. The seller isn't going to just offer a lower price, you have to ask! Or, better yet, just start negotiating. I used to think a good opener was "is that price negotiable?" but this gives and easy, closed answer of "nope". So now I just start negotiating as if it's totally acceptable (usually, it is!).