Buying online from someone with negative feedback? Feedback is there for a reason and although many will not leave negative notes, hopefully enough will to help you make an informed decision. The same is true for private auctions, I rarely, almost never. buy from someone who is holding a private auction. What are they trying to hide? Shill bidding is too easily done in a private auction. It's bad enough that eBay has changed the rules and all auctions are private over a certain dollar amount, but up until the amount, I want to check out the bidding status and bids.
Another thing that usually gets ignored is a seller's other jars or closed auctions. This is especially important when the jar is purported to be a rare one. Has the seller already sold three or four "rare" jars in the past? Perhaps it's not as rare as they claim or if the cookie jar is claimed to be an antique, bells should really go off!