When I first came on board as a staff member we had this same discussion. I'll provide some quotes from other staff to explain what we've basically come to an agreement on.

Originally Posted by Bazza` The general rule is any posts that detract from, add nothing or are just spamming a sale thread are not welcome.

Originally Posted by Big Zee So my stance on the matter is that I don't mind the "Good luck" comments - as long as they come with something contributionary to the thread.

Critiques are not allowed in the marketplace, but adding something useful to the thread is. Basically, if we, the staff, find something another member has posted as taking away from the sale or just not adding anything useful, then we will remove it.

It's somewhat difficult to just remove all the "Amazing design!" posts in the marketplace as not all are as simple as that. The staff will remove the comments that do not add anything useful to the thread. Keep in mind that bumping your own thread will result in an infraction, but I think Wildhoney said it best:

Originally Posted by Wildhoney Well as far as I can see, a thread is worth returning to the top if the seller posts something along the lines of: - Something else is added to the auction; - Bid notification; - Price reduction; - Answer to a question; - Important change in sale;

So to sum it up...
Specific critiques on how a design looks such as "I don't like the header links, you should change the color" comments are strictly prohibited. If the thread creator wanted feedback he would have posted in another forum.
Adding something useful to the sales thread is alright, as long as it is useful.
Bumping your own threads will result in an infraction.

Keep in mind that there were never any actual rules laid out regarding this. This is just my interpretation of what everyone said and how I feel we should handle it.