Where I am, $150 is a fair price for a treadle machine in cabinet in good working condition. In pristine condition, I would think $200 would be the highest price; absolute max would be $300.
If it truly is in pristine condition, I can understand the $300 price tag. However, then I begin to wonder if it could be a reproduction instead of the real thing. I don't know much about reproductions, so really don't know how you would check on that. Most of the vintage machines will show some wear-and-tear.