How is this app different then setting the high bit for automated bidding on ebay? And what if someone use the two at the same time?

You set your bid earlier, while sniper set your bid on last seconds so others will not have enough time to react on it.

Not sure about the second question.
One listing can accept several bidders at the same time. The highest bidder wins.
This has nothing to do with the sniper, that's how eBay works.