Mauer should be and is on DVauthrin (boys of summer)'s team. Because Mauer was traded last minute (after keepers were supposed to be submitted on yahoo) I couldn't drop and add him to DVauthrin's team. So instead I only kept 6 guys on his team and then added Mauer as a player he drafted.
So just like guys being drafted moving forward (IE McCutchen) they won't show in the available players list on yahoo, but if you search for them they show up as free agents until the draft results are 100% completed. Its a weird yahoo thing that's been around for forever.
If you want to draft a player, just ctrl f in the taken players and draft results thread to make sure they haven't been kept or drafted.