If they're retailers, they're not likely to take you up on your offer.
You can show them this and try the old, "they didn't even sell for $750" tactic, which gives you a little more credence.
Or, if you're passive like I am, you could just say, "what is your best price, including shipping and insurance?" That actually works sometimes, especially if you're polite about it. And tell them that "you're ready to buy now".
But someone with more experience negotiating online will give you a better answer.