Closing Belle's answer is correct, but there is an additional usage that might be helpful for your specific situation. I can't provide German equivalents, but hopefully these will help you somewhat.
A sales clerk is someone who works behind a cash register in a store; it can often also refer to people who also serve customers in a store. The term refers to a low level worker, in most cases.
A salesperson includes the above, but is also used to refer to those who sell goods not in a store-like setting and includes higher-level people. One can use this for anyone, at any level, that is in the business of selling things, z.B., traveling salesman, a merchandiser, a marketing representative, usw.
Additionally, (this may have significance in the case of the psychology article) "salesperson" is also used as a descriptor for someone who persuades, z.B., someone who is able to convince a person (or many people) to change their opinion about something (even something like a political issue or an initially reluctant new acquaintance to go on a date) is often referred to as "a good salesman/saleswoman", even though there is actually no commercial transaction involved.