look for 100% reputation, 99.x can consider too, sometimes they get -ve comments because the buyer got taxed by customs, which is a lame reason sometimes. the buyer should be aware of this himself.
i have purchased old items that are not easily found on forums, look for sellers with good rep. But if it is really too cheap to be real, then it is probably unreal..
As other have said, check on the reputation and only deal with those sellers that are at 99+% positive feedback and with a lot of transactions. Do look at the total amount charge since some seller have a high shipping and handling charge that negate the lower price on the item that they advertise.