There are two different strains of leatherback- codominant and recessive. If both parents were codom leathers, then this guy would be a leatherback as well if he carried the gene. If both parents were recessive leathers, then this guy would be a leatherback as well as a homozygous recessive breeding can only produce more homozygous recessives.
You might want to ask the breeders to be more specific about his lineage. Maybe one parent was a recessive leatherback and one was a normal scaled dragon? That would the only way he could carry the trait (at least at 100%) but not actually express it.
He's a good looking dragon and I wish you luck with the sale.