I personally think you guys are being a bit too hostile towards him. Where was this rage when people were spam posting rip off Mercing prices? If he wants to pay this amount, best of luck to him. It doesn't affect us if it takes him a while to find sellers. He could be buying individual plates, we don't know.
People are just jerks that's all. It's a low offer but theres always going to be someone who wants their Gil right now and not in one hour, and they'll take the price cut for the exchange of quick cash. Some people don't get all into that "economy/it will eventually sell" crap. They'll sell just to get rid of it and make some cash right now, or to help someone else. I know sounds like a strange idea but some people do that.
When I was buying hpb a few weeks back, I was offering some fairly low prices for bulk, like 5800-6000. I didn't expect to get many but a lot of people did sell at that price. The rest I bought for various prices. I'd estimate about 3000 hpb was bought underpriced. If the original poster buys just a handful of hmp for his Target price, he still makes out better than if he buys them all for the market price.
The offer is what the offer is. Either counter negotiate or ignore it.