I was a somewhat novice and nervous rider when I bought my first mare, a 4yo green jumper. Thankfully she's always had a good head on her and I matured as a rider quickly after I got her, but I know we had some scary problems at first, no matter how good she was most of the time. :/ Green and Green isn't the smartest combo. Also, beginner riders can ruin a good horse. Maybe they want to see him be used to his full potential. I know If I ever sold my mare, I'd want someone experienced or intermediate so she'd be used to her full potential. I'd hate to see her pack around a little kid, picking up bad habits.
“Have fun - Stay on top.”
"There's nothing like sixteen hands between your legs"