Half.com, is owned by Ebay. So your feedback will be the same for both sites. Just make sure you use the same username and password for both sites.
It is free to list on Half.com, but you will get charged a small fee when the item sells. This fee is automaticaly taken out of the sale price of the item. Also Half.com gives you a shipping allowance, that will be added to the price total.
So say you list a book for $5.00 and then shipping is media mail, they will charge your buyer like $2.00 for shipping. So Half.com will charge your buyer a total of $7.00, then they will take out there fee (and I can't think of the % right now). So you will never have to pay half.com's fees like you do ebay's fees.
The buyer pays Half.com, then when the sale has been O.Ked, Half.com will then send you an e-mail telling you that the item was sold and where you need to mail it.
You money will be held in your half.com account until the 1st or 15th on the month, then Half.com will deposit your money from sales on the site into your bank account, that you have provided to them, when you opened up your half.com account.
I hope this has been of some help. If not just post your other questions, and I will try to answer them.