The most important factor in determining the auction result is the presentation of your listing. I was going to say the image of the note is most important, but the image is one aspect of the presentation. Your note will sell based on how it looks. It it looks better than the assessed grade, the selling price will be very close to the grade or higher if there are at least two bidders who are prospecting on the possibility that the note is actually better. On the other hand, if the note looks lower grade than the stated grade, this will kill interest in your note, though you may still get lucky. Bidders don't want to waste their time pursuing a note if they think they will be up against other bidders who will be offering more based on the higher assessed grade. Grading companies tend to assess only technical grades that don't take into account the note's appearance. This is usually bad because many mid-grade notes look worse than their technical grade. A soiled VF will sell for much less than a cleaner, crisp-looking VF. Notes in AU and higher all tend to look the same, so there's no way for the bidder to know the true grade, and with big price differences between AU, Unc, and Gem Unc, a slabbed note with an explicitly stated grade will insure you get a better result. Raw notes in AU and higher will get much more wide-ranging results based on the faith the bidders put in your listing.
In terms of image size, bigger is not better. There's a reason why so many sellers use smaller images with high contrasts: it makes their notes look better. Actually, there are two reasons, the other being that a staggering number of paper money sellers are technologically incompetent and don't know how to use their cameras/scanners to achieve realistic images.
Item descriptions should be succinct. The more you say, the greater the chances you will say something that turns off bidders. Don't put your listing policies in your item descriptions, put them somewhere else. For instance, put the item description at the top, then the pictures of the item, then the policies below the pictures. Don't threaten potential bidders by saying stupid things like you expect payment in X number of days or you will file a non-paying bidder alert. Follow the KISS rule (i.e.,
Keep
It
Simple,
Stupid). Bidders who really want to know something will send you questions, so answer them directly instead of providing every answer to every unasked question in your listing. Don't overcharge for postage. Buyers overcompensate for inflated shipping costs in their bids. In fact, every dollar you overcharge in shipping can result in a $5-$10 loss on the auction result. But don't offer free shipping either because that comes across as desperate. Buyers expect to pay something for shipping because a seller taking money for shipping is like a guarantee that they will ship the item, whereas free shipping sounds a lot like no shipping, or slow shipping.
I offer this advice for free because I know nobody will ever use it
