On the PC, look for the blue “+” icon and “ウォッチリストに追加” on the right of each item’s page (the headline) to add items.

On the iPhone, it’s simple:
On every item’s individual page, there a “+”sign on the top right of the screen. Push it, select the top button (ウォッチリストに追加), done.

The watch lists are synced.

To view them on the PC, go to “マイ・オークション” on the top of the Yahoo auction page and select ウォッチリスト.

On the iPhone, select the star button on the bottom (the second from the left, マイオク) and then select the top button (which says ウォッチリスト).

If so, could you explain how I can add items to my watchlist on the pc and then view them on the iPhone?
I have a ya account so I guess that should be possible..