however.. it's ridiculous that this would flag an account on the same PC.. clearly a false positive.

It might be just coincidence; there are so many non-existent iphones(and similar desirable items) listed in this moment, it is possible that what set the alarm was what I outlined above, not because you were using different browser. 

If you are on the level, you could also offer as payment option "cash on pick up", that gives people more confidence that the item actually does exist.  That is cash on pick up NOT cash on delivery, which is AP service that you would never want to offer for anything.  Not sure if that would make eBay happier, but it may get people to bid more.