Best retort to "send your best offer"?

[Ben Ballinger]

Seems like every time I put an offer on a property that is less than asking price (or even when it is asking price), they come back with "send your best offer". I get it from their perspective, it's a smart response, but what's the best way to respond to this? If I just give them the best offer then it completely eliminates any negotiating ability, so is there a way to respond that opens up the discussion for negotiating back-and-forth?

[Roy N.]

@Ben Ballinger

We had a realtor come back to us last week with the same line ... so I resubmitted our offer 10K less than the first time.   

The realtor called back both a little confused and a little perturbed and asked why we would send a lower offer.   My response was that he did not indicate "best for whom" when he asked for our best offer.