be careful with any 'used chainsaw' and inspect it very carefully as used saws, no matter how used or not used they can suffer from a multitude of abuses, most common is the lack of lubrication from being straight gassed. Bars and loops can be easily replaced but internal damage is a different animal. As a rule I don't buy used chainsaws but..
The one thing I'd do before purchasing it is pull it over slowly (ignition off) and feel the compression and pull the muffler off and run the piston up and see if it's scored or has any unusual wear (and if the owner has nothing to hide he should not object to that) as removing and replacing the muffler is easy and won't impact the saw in any way. If he or she is hesitant about that, it will tell you the saw has either been straight gassed or has been abused in some way. The piston won't lie. and if it is scored and you aren't adverse to rebuilding it and have the tools and expertise to do it, that makes the asking price extremely negotiable.
400 bucks is 400 bucks anyway you cut it. Just my opinion, take it for what it's worth and proceed but understand, internal damage will always be a deal breaker in my view and why I never buy a used saw. Chainsaws are usually flogged pretty hard and not maintained properly, generally. if I want to flog on a saw, I prefer starting out with a new one, versus used and internal damage to the piston, rings and the jug always tells the story and observing don't cost anything and can make any asking price negotiable.
of course you won't be able to observe the jug but if there is scoring on the piston, you can bank on the jug having at least some metal transfer from the piston and it only takes a couple seconds of straight gassed or inadequate lubrication to damage any piston.
Just my opinion and you proceed how you want to.
There are always the clone saws (Chinese copies) if you want to go cheap nut be apprised that what may appear to be a bargain may or may not turn out to ne one. I bought one some time back (a Farm Tech) and it turned out to be a rock and I gave it away to a not so close friend. It ran fine but build quality was sorely lacking.