I'm relatively new to the game so maybe there are some restrictions I'm not aware of, but I can't bid on any item auctions. When I go to the page, the "Place Bid" (and "Buyout" when available) buttons are disabled no matter what I input in the "bid" box.

Zeerose, do you have free space in your inventory? 

Oh, I don't. That explains it. I feel so silly now, thanks!