Once you click the advertise button, it puts your item at the top of the list to be displayed on the marketplace. The flaw is that, during peak hours especially, so many other people are advertising their wares that your items are quickly moved down the list and out of sight. Your advertised items are never recycled through the list and will forever continue moving downwards.
The trick I've found is to list the same item (even if it is just one and not a stack) at a low price to entice buyers to visit your city and they then see that previously posted item once they're in your store. The chances of them buying both are high. I've also found better chances of selling in off-hours (late at night) rather than at 8pm when everyone and their mother are posting.
