Well, people don't realize that posting an advertisement with untested hard numbers is a legal problem. Aftermarket part marketing is good at implying huge improvements while claiming nothing, and some customers assume the best. Installing 10 parts that all claim "Adds up to 25hp!", a user could naively assume they added 250hp.
When buying a used car, there's some caveat emptor as well; check it out before buying. Asking how a Sportage SX is 300hp is a reasonable question.
Best of luck with the car sale, SnakeSX.