Rounding would apply on the total transaction, not individual items (because otherwise the individual posted item prices would just be false.) So, if there is an abuse route with round-half-down, it is that optimizing buyers would structure purchase to always total $x.01 or $x.02, possibly splitting planned purchases into multiple purchases to achieve that.
But even that isn't realistically a significant issue.
Rounding would apply on the total transaction, not individual items (because otherwise the individual posted item prices would just be false.) So, if there is an abuse route with round-half-down, it is that optimizing buyers would structure purchase to always total $x.01 or $x.02, possibly splitting planned purchases into multiple purchases to achieve that.
But even that isn't realistically a significant issue.