I wish I knew how to judge a player (especially a young player) based solely on attributes, instead of being seduced by misinterpretations of age, DMI, and potential. This could have helped me.
I wish I knew the severity of the percentage hit on your sales, and that making small sales isn't worth the tax in the long run.
I wish I knew how much the market would change, and just saved up all my money in S12 and built a super team when prices plummeted.