The site you linked is asking crazy prices. For example, a clipped signature like the one I linked is over $200. (That's ten times more than the equivalent eBay
listing, which did not even sell at $20). As you know, asking prices mean little. Folks can ask whatever irrationally high price they desire.
We have not even seen the entire postcard in question yet. The value of this postcard is unknown until we can see the entire thing and then find comparable 'sold' prices.
I agree that the card sounds interesting and is probably worth more than a typical postcard if legitimate. If it contains unique historical comments, perhaps even worth a significant amount. (It could be noting that he is bringing plenty of laundry home from the games for all we know.) So in my opinion until we see the entire card and can do discovery on realized prices for similar items, it is prudent to not set high expectations.