what if people see this
Operative word is "people" - plural - if there is only 1 interested buyer then the price would go low, but somebody who really wanted it would not take a chance on missing it and buy high.
To avoid that a seller puts it up for auction and takes his chances.
The psychology of selling is not a science.
Emotionally, I am a person who is more comfortable paying more for a "sure thing" if it is something that I really want.
Auctions are where I go to lowball things that I am indifferent to.