Merchandising income comes from your fans buying team apparel. Jersey sales are affected by a combination of factors. One factor is whether you have a winning team. Nobody likes to wear the jersey when you're losing, do they? Other factors include how many domestic players you have on your team, and also how many were drafted by your own team, as fans like supporting local players more than imported talent. This is weighted by the overall skill of the local players, so stocking your team with local bench warmers won't do you much good.