Well here are my examples:
My merchandise in the offseason when none of my NT players was playing
5/28/2012 Weekly Merchandise profit $ 97 847
6/4/2012 Weekly Merchandise profit $ 97 551
They all started playing within a few weeks (playing each week, as i agree playing is important, but minutes played has a far lesser impact if at all)
6/25/2012 Weekly Merchandise profit $ 135 020
7/2/2012 Weekly Merchandise profit $ 131 983
I have 3 NT players, 2 from my own nationality (Macedonia) and another one from New Zealand. My own nationality NT players total 200k in salary (130+70), whereas the foreigner was at around 135k. The total increase was at around 35k and i've been at the same number ever since.
One possible explanation could be this (from the game manual):
fans from around the country are particularly enthusiastic supporters of members of their national teams, and even more so when their national team is a successful one. So, if in a given week your player played in a national team game, fans from around the country will buy their jersey, typically in a quantity proportional to their salary. National team jersey sales will vary depending on the strength of the national team, but players playing in their domestic league tend to sell considerably more national team merchandise.
So maybe the success of the national team makes a huge difference?