I still think that separate forums is by far the best way to go about this.
Maybe not Subforums like I first suggested but actually separate the Buy and Sell forum into several forums.
Examples,
- Buy and Sell Forum - Content
- Buy and Sell Forum - Domains
- Buy and Sell Forum - Traffic
- Buy and Sell Forum - Other Services
Allow 1 bump once a day or in a 48hr period.
Also it would be good to allow members to close their threads once their items are no longer available.