For example, an in-country advertising or public relations agency can validate or disprove your assumptions for that market. National laws and regulations. Your site has to deal with laws that regulate how your product works, how you sell it, and the tariffs, taxes, privacy laws, and other minutiae that affect its sale in a given market. And those nits can be pretty arcane. Unfortunately, there's no international registry of all laws that could affect your plans for global domination.
Sites can provide some insight on the privacy laws in other countries.
Local business practices. Less obvious than national laws and tough logistics is the way of doing business in a given country. To compete effectively in Korea and Japan, you may have to align yourself with the most appropriate group. For example, Brazil might look like a less desirable market if you consider nothing but percentage of the population online. However, once you exclude rural areas and the rain forest, focusing instead on the densely populated cities along the Atlantic coast, Brazil becomes a much more attractive online market.