Purchased/traded - this has come up before... I'd like to stick to "traded," since that's what we've been operating under and we already have some teams in the league. We can debate the question in the league message board once the season starts, but for now, let's stick to traded, not purchased.
go_pre... Correct. You can have those guys on your team, but that particularly trade - in that particular direction - cannot be used again.