That's been up for awhile. 9 offers unaccepted. It is a great price, but the interest in the correspondence in general
seems to be limited. Also, that Martinelli book is a bit of an outlier. But the price is more than fair.
Oh well, I hope the buyer enjoys them. The letters are some of my favourite books and I think they are undervalued (I mean, readingwise not just pricewise). I wouldn't know about prices because I'm not a collector.
$400 is just about market for those. The signed Screams is a $200 book; $75 each for Living on Luck and Reach for the Sun, and $50 for the Martinelli is about right. So, depending on how much lower that $400 the accepted offer was, there's your bargain margin, as it were.