I understand what you are saying, but look at it from this point of view, to give some of these "positive" comments is like a "kudos" for what they are selling. It is better, sometimes, than putting your books up for sale and get no response at all. You can't tell if your books aren't worth it or if maybe people just don't have the dough to spend right now.
Just another way of looking at it.
... and as an added bonus, those little comments show activity in the thread. Always makes me think there is a party going on there and I need to look.
The more people that look... and then re-look... just improves an item's chance of selling.
But what do I know... I sell pure sheeeet