Now for a very historical trade - the very first one in major league history. Retrosheet’s database lists July 20, 1886 as having the first trade ever:
Baltimore Orioles traded $1000 and two players to Lawrence (New England) in exchange for Pat O’Connell, Dick Conway and Bill Conway.
Before that, there were multiple transactions that were essentially trades for cash only, but this is the first one that included a player going in each direction. Unfortunately, there is no record of who the Orioles sent to Lawrence. It does not appear that any of the three men acquired by Baltimore played for any other team after the Orioles.