"That Jesus' resurrection was the very center of early Christian faith..." doesn't necessarily count towards its status as fact. If a religious sect is being started it needs something to make it special, and a resurrection will fit the bill. The emphasis on the resurrection could have been something early Christian leaders promulgated in order to gain followers, regardless of its truth value. (I'm not saying here that those leaders were deliberately fraudulent, but they would have been aware of which aspects of their faith would be most persuasive to potential converts.)
Habermas uses (and excuses) multiple appeals to authority:
Throughout this essay, I will not assume the inspiration or even the reliability of the New Testament writings, though I think these doctrines rest on strong grounds. I will refer almost exclusively to those data that are so well attested that they impress even the vast majority of non-evangelical scholars. Each point is confirmed by impressive data, even though I can do no more than offer an outline of these reasons.
We must be clear from the outset that not only do contemporary scholars not mind when points are taken from the New Testament writings, but they do so often. The reason is that confirmed data can be used anywhere it is found.
Habermas's case appears to be a "resurrection of the gaps" argument. He claims there's no natural explanation for the post-mortem appearances of Jesus, and that therefore Jesus rose from the dead. First, this is an argument from ignorance, and second, the burden of proof is on those making the extraordinary claim. The evidence from scripture is shaky at best: note that there are no eyewitness accounts of Jesus actually rising from the dead — it's all post hoc supposition, with Habermas and other apologists filling in the gaps themselves with events they want to believe happened.
It's not up to anyone else to disprove the resurrection, because it hasn't been sufficiently established to begin with. I can't account for what goes on in the mind of a religious zealot, but I am highly suspicious of any extraordinary event reported by eyewitnesses. Eyewitness accounts are notoriously unreliable, and if those accounts are mostly reported second-hand by one individual with his own agenda we have good reason to be skeptical. I may not personally have a definitive explanation for the scriptural accounts, but I don't need one. As far as I'm concerned there's nothing to disprove.