I need to know which countries in Central America (I am not fantastic
at geography, so if Mexico is not included in that question then I
need that country as well) require you to use agents or trading
companies in their country in order to do business.
  
Thank You,  
Joel

According to my research, only Nicaragua requires an agent in all
cases. Costa Rica requires one in certain situations but the rest of
the countries tend to follow US practices.


Belize - not required but it is recommended: "Because the local
economy is extremely small, the best way for American exporters to
penetrate the Belizean market is to seek a local importer/wholesaler
who would generally act as their agent/distributor in Belize."
http://www.usatrade.gov/website/ccg.nsf/CCGurl/CCG-BELIZE2001-CH-4:-0063BBA0

Costa Rica - only required "for those companies wishing to participate
in Costa Rica's government procurement system."
http://www.usatrade.gov/Website/CCG.nsf/CCGurl/CCG-COSTA_RICA2002-CH-4:-0068503C

El Salvador - not required but an agent is commonly used: "When a U.S.
firm wishes to sell in El Salvador, it usually appoints an agent or
distributor."
http://www.usatrade.gov/website/ccg.nsf/CCGurl/CCG-EL_SALVADOR2002-CH-4:-00425112

Guatemala - not required but keep in mind that "Most business is
conducted in Guatemala based on personal relationships." and "U.S.
suppliers should be prepared to have a local representative or
distributor and to travel to Guatemala personally."
http://www.usatrade.gov/website/ccg.nsf/CCGurl/CCG-GUATEMALA2002-CH-4:-004C4B2D

Honduras - not required but "To market aggressively, U.S. exporters
should establish local representation or a local sales office."
http://www.usatrade.gov/website/ccg.nsf/CCGurl/CCG-HONDURAS2002-CH-4:-00636894

Nicaragua - required: "Distribution and sales of imported products are
handled by local distributors and agents."
http://www.usatrade.gov/website/ccg.nsf/CCGurl/CCG-NICARAGUA2002-CH-4:-00530F3D

Panama - not required, although commonly used for industrial goods:
"For industrial goods, sales are normally handled by local exclusive
agents or distributors."
http://www.usatrade.gov/website/ccg.nsf/CCGurl/CCG-PANAMA2002-CH-4:-00492071

Mexico (which is part of North America) - not required but could be
helpful: "some Mexican firms are interested in serving as sales agents
for American firms. This channel can be efficient for reaching the
smaller cities or more remote locations of the country."
http://www.usatrade.gov/website/ccg.nsf/CCGurl/CCG-MEXICO2002-CH-4:-0054F200

***
Summary:

Belize - no 
Costa Rica - sometimes
El Salvador - no
Guatemala - no
Honduras - no
Nicaragua - yes
Panama - no
Mexico - no

Let me know if you need clarification on anything!

liza-ga

Thank you for the answer and the summary for each country researched.

Joel