Whether it's new or not, I'd avoid it. First, the phone could be reported lost or stolen, so it may be blacklisted in VZW's system, which means you can't activate it. You could ask for the ESN so you could check it with VZW, but you have no way to know if the one the seller gives you is the one stamped on that particular device. Second, even if the seller appears to have a good rating, and the account has been around a while, it's possible the account has been compromised and hijacked by a scammer. That's how they operate: find an eBay member who either has a weak password or who falls for a phishing e-mail, steal their account, then offer phony products for sale.
And, just for the record, those phishing attempts are pretty slick. I sold an item a few months ago, and I received an e-mail saying that a potential bidder had a question, and, naturally, it provided a link I could click to log into eBay. The message even listed my item's auction number. What tipped me off that it was bogus is that it was time-stamped after the auction had ended, and the login URL wasn't to an eBay domain. Still, it was pretty convincing, and it would have been even more so if it had come in while the auction was still open.