By Del Quentin Wilber Los Angeles Times SAVANNAH, Ga. — When Secret Service Agent Matthew Britsch began trawling for major counterfeiters in the shadowy marketplaces of the dark web, he acted like any smart consumer on eBay — he studied the reviews. Britsch knew he had struck gold when he found Billmaker, the online moniker of an anonymous counterfeiter who promised a high-quality $100 bill and a money-back