The "Make an offer" function is turned on by default for every listed item. Once your item is listed, you can choose not to receive offers. Here's how:
1. Go to your "Account"
2. Select the "For sale" thumbnail
3. Go to the "Current negociations" section
4. Select "Settings" and select the items you don't want to receive offers for. You can also choose not to receive any offers at all.