From the game manual:
http://www.buzzerbeater.com/community/rules.aspx?nav=Econ..." 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 yes, strength of the national team does make a difference.