Нашел всего пару примеров употребления "up-market retail customers":

Our Vision
To provide superior proactive insurance and investment services to niche market globally, while providing cost effective responsive services to others in our role as insurance and investment agent and in doing so meeting the needs and wants of our clients. To become the agent of choice for others and for the corporate, medium business and up market retail customers. To provide cost effective development insurance and investment schemes for the people, the business and mass market.
http://www.anmollifecare.com/aboutus.asp

Our Vision
"To become the bank of choice for corporates, medium businesses and up market retail customers and developmental banking for small business, mass market and rural markets."
http://www.boikenya.com/aboutpage.html

"Up-market":

Upmarket (or high-end) commodities are products, services or real estate targeted at high-income consumers. Examples of products would include items from Mercedes-Benz, Samsung, Hammacher-Schlemmer, and Chanel.
http://en.wikipedia.org/wiki/High-end

"Retail customers":

RETAIL CUSTOMER – Any customer other than an institutional customer. Retail customers generally include individual investors and small organizations. Compare: INSTITUTIONAL CUSTOMER.
http://www.msrb.org/msrb1/glossary/view_def.asp?param=RETAILCUSTOMER

The deal increased Bank of America's presence in Illinois, Michigan, and Indiana by 411 branches, 17,000 commercial bank clients, 1.4 million retail customers and 1,500 ATMs.
http://en.wikipedia.org/wiki/Bank_of_America

Исходя из всего этого, получается, что "up-market retail customers" - это состоятельные клиенты.