I'm a social scientist in an economics department in Britain. Two of my interests are experimental economics and on the social causes of genetic variation. I also have a newsletter at https://wyclif.substack.com and am writing a book at https://www.wyclifsdust.com
Can't believe nobody's mentioned Pascal's wager. Surely this is the simplest reason not to sell your soul.
The other reasons seem to me like the irrational tail wagging the rational dog. If you are sure you don't have a soul, then selling it for $10 is not a big deal, just as if someone offered to buy my Thetan and I'm not a scientologist.