Partially it's due to that gray area surrounding "illegal" and "against FF program rules". As Ebay says:

"...the seller may play no role in arranging any travel other than delivering the item. Please note that many airline tickets, vouchers, and frequent flier miles are not transferable, and sellers should review any travel related item before listing to make sure that it can be transferred legally to another party and will be accepted by the issuing provider of travel services."

The other part of the answer is the volume that Ebay handles. There is no practical way to review every auction, until someone complains. Again, Ebay says:

"If a listing is ended or removed, it's probably because the item was prohibited, questionable, infringing or potentially illegal as stated in the User Agreement and a specific complaint was reported. Listed items similar to the ones removed will remain on the site unless eBay is notified about them." (emphais added)

Basically, the airlines have to complain about every listing in order to get each one removed. Then, of course, Tino (or any seller) can just relist the item without consequence. That's why the airlines like to wait for the auction to end and burn both the seller (by taking away his accumulated FF miles) and the buyer (by cancelling the ticket purchased). Clearly, the idea isn't to catch everyone, it's to scare people away from these types of transactions.

Personally, I'm one of the folks who say "these are the rules that I agreed to (not selling miles or awards) and just like I expect the airlines to live up to their side of the bargain, I need to live up to mine." I do believe that when an airline increases redemption rates with little notice (say for a RTW ticket), that the person saving for such an award may have an ethical opening for disposing of the miles accumulated toward that reward. That's far different that running up gift card purchases on charge cards for the purpose of selling the miles or awards.