selling share

One of our shareholders wants to sell his share to another person.
Here are more details: С-corp registered in New York, NY Founded: June
2010 Business description: This company acts as a legal/sales proxy
for a software development business located in Russia and Belarus that
exists since 2003. All management and sales is done from Russia, and
actual work is performed in Russia and Belarus. Customers are in the
US and Europe. Employees: not at the moment (one employee used to work
for 3 months) We rent an office in New York for temporary visits of
consultants from Russia. One of the shareholders, who owns 51%, wants
to pass 25.5% of his shares to a third party. We want to make the
transaction as simple as possible with as little expenses (both taxes
and fees) as possible. The main business is in Russia and it has
already been sold and all the money issues settled. What is the best
way to process this? Being a non-resident does the shareholder have to
pay any taxes in the US (we want to minimize it to zero, but IRS may
object and require "fair" valuation)?Best regardsAna