I respect the fact that you believe in this idea enough to suggest it here on an open forum, however, others feel much diffferently about it, and many are not in favor of such a notion.  For one thing, the buyer in a transaction would naturally wait until their product arrived, and then look it over and decide if they liked it or not.  Most of the time they do.  But at that point there is nothing you could do to force them to go back to the computer and waste time to add a comment - unless you blocked them bidding again until they did.  

This would only anger the buyer, and you know about how ebay feels about buyers - they are gods!  This is not going to happen!  Trust me about this!

And, if you did somehow force such a notion through the system, the buyers would retaliate with horrible, horrible feedback!  It would be the sellers who suffered from this one.