The solutions are saying that 'han vil gerne sælge' and 'han vil sælge' are equivalent. Logically, these seem different to me--the former expressing desire, and the latter subjunctive.
"Vil" can express desire on its own, in Danish. I think it's more a matter of the tone in which it's said, which is harder to describe through text
if you include gerne it is more polite but the meaning is the same.