There are a few different ways you can make an offer.
1. Click on offers under each post. You then type in your offer along with a working email address for you. After you are done typing, go ahead and click on post a comment. If you are familiar with the blog world, it is just the same as leaving a comment on a post. The Owner will then receive the offer into their email.
2. Just send the Owner an email directly and work something out that way. All ads should have an email listed for the owner.