How does courier-sharing help reduce pollution ?

If you buy something in the small ads section of a paper or on a site like eBay and you use your car to go and fetch it (let's say it's 25-miles away), you'll have put 50-miles worth of CO2 into the atmosphere. CO2 emissions per km produced by a car 
On the other hand, if the parcel's taken from A to B by someone who is in any case making the trip, you'll have made your own small contribution to cleaning up the atmosphere.
  
The information and data given below can be found on the website of Ademe, the French Environment and Energy Management Agency:

French Environment and Energy Management Agency

The road-transport sector is responsible for over a quarter of all greenhouse gas emissions in France. It is therefore highly important that people restrict their travel to essential trips and encourage the use both of public transport and of "clean", environment-friendly vehicles.
Fuel consumption and CO2 emissions..
The combustion of fuel in a an engine produces various gases, the main one of which is carbon dioxide (CO2). This gas is one of the main constituents of the greenhouse gas effect, which is contributing to climate change.
Following the objectives set out in the Kyoto Protocol, France has undertaken to reduce vehicle fuel consumption and emissions. A voluntary agreement to reduce vehicle CO2 emissions has been signed between the European Commission and the car manufacturers operating in Europe. Under this agreement, CO2 emissions are to be reduced by 25% from their 1995 levels by 2008 (to 140 g/km as opposed to 186 g/km), with a further reduction to 120 g/km (35%) by around 2012. In 2006, mean CO2 emissions in new cars sold in France were 149 g CO2/km, which puts them in class D of the energy/CO2 label.
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ADEME
ADEME actions as regards transport :
 
Transport less” by reducing traffic volumes (number of trips, driving distances, etc.).
Since May 2006, the colour-coded Fuel Economy and CO2 Emission Labelling Scheme has been obligatory in France on every new car purchased in the country. In the UK, the scheme is still voluntary, but most new car dealers now display the label.

The label is similar to the familiar colour-coded labelling for electrical appliances. It tells potential buyers how much CO2 the car in question emits.
Carbon dioxide, or CO2, is the main greenhouse-gas responsible for climate change.

A vehicle's fuel consumption and CO2 emissions depend not just on its fuel efficiency but also on driving habits and other non-technical factors.
Atmospheric pollution is one of the major concerns of the new century and it can be grossly divided into three types: local regional and global, each of which has different effects on people and the environment.
Regional pollution is caused by the spread of pollutants over distances of up to several hundred kilometres from their source. Their action depends on meteorological conditions, such as rain, wind and sunlight.
Global pollution is the cause of problems that affect the planet as a whole, such as the destruction of the ozone layer at high altitude, or the enhanced greenhouse effect, believed to be behind the recent warming of the Earth, with its accompanying adverse climatic effects.
Local pollution is caused by waste and discharges from sources such as industrial plants and transport corridors, from which are emitted primary pollutants, such as carbon monoxide, unburned hydrocarbons, nitrogen oxides, sulphur dioxide and fine particles suspended in gas.
Car pollution
Although pollutant emissions are being brought down in France in most sectors, transport-related pollution is still on the rise. The situation is particularly worrying in built-up areas where motorised transport is the second biggest source of nitrogen oxide and particle emissions. It is also responsible for a significant percentage of carbon monoxide (CO) and unburned hydrocarbon (HC) emissions.
European standards on emissions of private cars
Every 4 or 5 years, emission limits are tightened and we are now on the Euro 4 standard.
Progress
Increasingly strict regulations, coupled with the technical advances made by car makers and equipment manufacturers and also by oil companies, have resulted in progress on a number of fronts.