First, Cory Schwartz of GameDay reminds me that they use MLBAM IDs, not Elias
They have unique ID numbers for persons (players, coaches & umpires) and for teams and ballparks. I would recommend using these and not any autogenerated number for the unique key for these tables, and for foreign key references from other tables. If user auto numbers are used, it makes it impossible to merge sets of data, as the numbers are repeated from one set to another. Since they are already providing use with unique identifiers, it's good practice to use them for the primary keys.