Have in mind a price you won't mind pay for an item. Sometimes if you offer a price that is just too low and walk away, the seller might actually not call you back. And when you do go back, you might not get a price anywhere near what the seller mentioned before. So when you walk away, make sure you really are willing to walk away.

Suggest a discounted price if you are buying more than one item.

Don't be afraid to point out flaws (scuffs, unevenness, discoloration), anything that might get the seller to lower the price.

Just some things that have worked for me.