I'd be much more interested in their feedback, especially for items they've bought in the past year. An APO by itself isn't indicative of a scammer, but if you feel uncomfortable, and the auction hasn't ended, don't be hesitant to cancel the bid. I cancel bids for anyone with a feedback less than 10, or with any reasonable neg on their account in the past few months.
You have to accept that any buyer can claim that you sent them an empty box, and there's little you can do to dispute it short of producing a video of yourself sealing and mailing the item, so there's a certain amount of risk with every transaction, and the best you can hope to do is minimize it. If a person has a long history of positive feedback, they're probably not out to scam you.