I wasn't comfortable providing a serial number ahead of time simply because someone could register it without physical possession of the computer. Is that a big deal? No. Might it complicate the eventual sale to a real buyer? Maybe. You see people posting pictures of the MBP box labels all the time on eBay, with serial number...so I may be paranoid here.
I let a 256 go for $1650 after over a week of jerking with Craigslist. Having bought and sold MBPs on CL for well over a decade, and occasionally made money on the transaction by selling an older model to someone willing to pay top dollar and buying a newer model at retailers with good prices, I can safely say that it's become much harder to get high values for laptops in the past few years. So this may have just been an artifact of that trend rather than people being suspicious of it being new. But in the old days, I could have fetched $1850 for it easily (and yes, that's despite several retailers selling it for $1799).