I've never done it, but according to their FAQs, here's how:
<<On very rare occasions you may have a problem with an item that you receive and may need to contact the seller. If the item that you received is not of the promised quality, was damaged in shipping or was different than the item description you can "ask seller a question" to resolve the problem. You would do this by finding the transaction in the Your Purchases section in your account and clicking on the transaction. From here you will be able to contact the seller. You will then be able to choose a problem type and send this message to the seller. You should receive a response from the seller within 2-4 days. The response will offer either a resolution or may require assistance from Half.com Customer Service.>>
*Edit because I had it backwards the first time and gave you instructions on how to contact the buyer.