As a fellow seller, this is a common practice. We all want to get our tickets sold, having the tickets in the front of the line is a way to go. Since it is an open marketplace, this is not illegal by any means.
Hey! You also have to keep in mind that there are dedicated large selelrs who use automated systems to aid in their selling processes. The continuous price change you experienced may just be a result of said automated systems. Definitely not illegal but surely efficient!