One easy way to call someone on their bluff, with less risk of being denied, is to show up in person and make your lower offer there after inspecting the item. People are much less likely to decline if you are right there and willing to walk away, especially if they don't actually have any other offers. "$700 and we can take it today", etc. I think seeing something walk (or be willing to) makes them second guess their price, as they can visualize being moments away from having the cash...
Another thing to consider, is that there is a lot of value to seeing/purchasing a unit when it is still installed, rather than a jumble of bits in a garage, or worse, having the seller deliver it. You get to hear how it runs, see how well it cools, etc, and also have control over how carefully it is dismantled and removed, especially for all of those trim parts.