half.com is an Ebay company. All the transactions I have done (and unfortunately I've probably done to many) have been great. Only once did I have an issue, my wife bought software and they shipped her the wrong one, but the seller fixed it all on her dime.
Over the years I've probably bought 20+ books, 3 or 4 movies, and a bunch of PC software. (my daughters got hooked on Nancy Drew PC games, and with over 20 games in the series at $20 a pop... I've had to find a cheaper/better way of feeding their habit...)
I have had a few great scores, saw a book that caught my eye but was $29 at a large bookstore, found it new on half for $10 delivered.
Shipping can add up if you buy from multiple sellers, so just keep that in mind. Some sellers will adjust shipping costs if you purchase multiple items from them.