In my opinion, ebay links should be allowed, it offers a relatively safe way of making the transaction, knowing that you have some form of protection.
Unless David would be willing to provide some form of cover for the members here making a transaction then it is pretty safe. I am not advocating that this needs to be done by David, but if members do use this area and get ripped off, then it is entirely at their own risk.
I believe allowing links to ebay auctions, provides an element of safety to doing this.
I also feel that there should be a minimum length of time that a member should be here, before they are allowed to sell or buy items, this would go some way to prevent spam or scam posts.