Wary = cautious, concerned
Leery = cautious, concerned
Weary = tired
You can state in the auction text "Bidders must have a minimum feedback rating of X number and Y % positive. Bids from people who do not meet this criteria will be canceled, and if the high bidder at the end of this auction does not meet this criteria then they will receive negative feedback and their bid will not be honored."
Putting that kind of text in covers you contractually in a way that Ebay's own policies do not.