This question involves two aspects: a) determining how many credits you may have and b) how do companies go about selling credits. Both parts are interesting, and can be tricky.
First, credits you could generate to sell need to be determined by a carbon assessment. The state of California, for example, has adopted a 'Carbon Calculator' that my research group, the Renewable and Appropriate Energy Laboratory (http://rael.berkeley.edu), developed.
We built a first version that is online at http://bie.berkeley.edu/calculator.html and a California-specific version on the state-supported website http://www.coolcalifornia.org. This one includes goods and services, energy, and vehicle purchases. Later commercial-sector versions will show the greenhouse gas inputs to energy systems, including wind, natural gas, coal, and a range of other technologies.
The issue in having credits to sell is that they must be not just things you might do anyway (e.g. buying a hybrid car to save on gas), but actions that would not have taken place in the absence of the incentive that the market provided. A typical U. S. resident, as you can see in the calculator (http://bie.berkeley.edu/calculator.html), produces about 40 tons of carbon dioxide per year (tCO2/yr).
Businesses often determine their 'baseline', or 'business as usual,' emissions, and then reductions from that count as emissions that could be allocated or sold.
Large companies often need tens to hundreds of thousands of tons of emissions credits, so buying from individuals is logistically cumbersome. A more likely approach is that individuals and small businesses would aggregate their credits (possibly through a company or community group that specifically does that task), and then those bundled baskets of credits are sold to the large company.
For those wanting to explore much further, a directory and a comparison of many different calculators is available online at: