Complex sentence: Historically IBM was used to selling products that were developed over several years and which cost as much as the annual sales of a successful microcomputer company, they had to learn how to quickly mass-produce and market new computers.
Split into simpler sentences: Historically IBM was used to selling products that were developed over several years and which cost as much as the annual sales of a successful microcomputer company. They had to learn how to quickly mass-produce and market new computers.