The quick answer is ...VERY !
You wouldn't walk into a shop with your eyes closed and pick out the first thing that you touch and buy it would you?
You need to pick it up, examine it, feel it and think about it but most of all see it. Ok so you have a picture of your item but how much information does that actually give a buyer ?
A picture of a poor conditioned item can look like new in a photo so don't trust a picture alone to sell your item.
I find a photo almost never shows cracks, rips, general wear and tear or other minor bad points. Describing an item well and honestly can really help the buyer make an informed decision.
The description lets your buyer know exactly what they are buying, the better the description the more chance you have of selling the correct product to the correct buyer.
If you describe your item simply and accurately the buyer will have no surprises once the item is received and your feedback will be good, but... if you have missed out the negatives of the condition of the item, your buyer is going to be angry and let down, and there feedback will reflect this.
Always describe items exactly as you see them, you are the eyes of your buyer and they're going to see your item eventually if they buy it anyway. ( remember...surprises are bad ).
Describing the negative points of an item in my experience does not put off the buyer, most of the things i sell are second hand board games and you have to describe the condition of boxes, pieces, cards and parts ect.
With a new game its not so important as the listing will read mint/new or unused but a game from 1970 will have creases to boxes, rips, warping to playing boards, splits and broken bits ( maybe ).
Don't go over the top with your description, keep it simple, accurate and honest.
I always describe EXACTLY what i see, a good guiding rule is to air on the side of over negative as when the buyer receives the item they are pleasantly surprised that things are not quite so bad as they expected.
You will not get anywhere at building trust in buyers by only focusing on the positives, in fact its a sure way to attract a bad feedback and keep repeat buyers away.
Hope something in there helped you, please don't forget to vote... and be honest !
All the best from ade@godsgames