On December 4, 1886, the Maroons traded Alex McKinnon to the Pittsburg Alleghenys for Otto Schomberg and $400. Three months later, Schomberg would be sold to the Indianapolis Hoosiers.
Make sure to take some time today to remember the immortal Schomberg for McKinnon deal. And please celebrate responsibly.
Update: It was brought to my attention that the McKinnon/Schomberg deal was the first player trade between National League teams (see comments). For an overview of 19th century player transactions, you should check out David Ball's work here. Also (and I just noticed this), Schomberg wasn't exactly sold to Indianapolis in 1887. The Black Diamonds folded, Indianapolis was admitted into the League to take their place, and many of the Maroon players ended up there.