Keller Sports, a German-based online retailer specializing in running, fitness and tennis, has received a new seven-digit financial investment from Christian Mangstl, who co-founded Scout24, a German online marketplace for the sale and purchase of cars, real estate, financial services and other types of items. The additional money will be used to hire new staff, improve services and finance Keller Sport's international expansion. The company opened new online shops in Denmark, Sweden and Switzerland last December.