Persuading someone to buy is a complicated process. Why is it? Is the process inherently difficult, or is it since so many people are included?

Persuading is an affecting skill where you seek to acquire someone to agree along with your proposition by showing that this is what is required. This is complex since there is never generally just one buyer. Classically a buying decision for something is as important as a systems project which will involve finance people, technical establishment, the eventual user and the project sponsor. Probably a stakeholder analysis will show that here more stakeholders, but they might not be involved into the purchase decision.

The process is complicated and this involves many people.