Whether I buy new or used, I try to save substantial amounts of money, to cover myself for the potential that something is wrong with the saw.
I generally assume that the saw should be ~40% off even if it's in very good to excellent condition.
I see beat up 372s all the time for $400--half of them are from a time when those saws were $650 at dealers--and I quickly pass.
Most of the time when I meet someone from craigslist I just run the saw, idle, accelerate, and feel the compression at the pull cord. Lots of sellers don't feel particularly comfortable with you taking the muffler out, hooking up all sorts of gauges and stuff to the saw, etc, and to a degree I can respect this.
Unless you find something that's poorly labeled, mislabeled, sold by a nitwit, etc on ebay, you're going to be paying market rate and then some.
At the end of the day my attitude is just to make sure that you have quite a bit of padding versus the cost of new, so that even if something does go wrong, you can probably repair it, unless the whole thing doesn't explode, and still be at or below the cost of a new one. If you buy a like new 372 for $600, and it's out of warranty, and you have a leaking boot or something, and then all of a sudden you're whole top end is fried, suddenly if you go OEM you're probably north of $900, and you got one expensive 372.
The other thing about Ebay is youre competing against people who may be used to paying more money for saws than you. I guess this is true for craigslist too but...at this point I know of a handful of husky dealers where I can save between 50 and 100 bucks of MSRP. When you're competing wtih some guy from wherever, and they're used to paying full sticker and don't know that dealers are out there (maybe theyre not in his neck of the woods), suddenly your $200 savings is a $300 savings for him, and so he's going to bid it even higher.