Does the listing date change when they're bumped? If the list date doesn't change, I'd assume it does not reset for auto since the sale is simply getting bumped, rather than fully relisted, and auto-purchase probably wouldn't pick it up.

If I'm understanding your question correctly. I've not used the bumping perk, and haven't the experience with it.

I haven't tested this much, but I got the impression that bumping doesn't trigger auto purchase. I had to cancel my sales and re-place it for this