It doesn't quite work that way. The $50K is taxed as capital gain - long-term or short-term depending on the holding period for each security. under the MTM election gains and losses are ordinary not capital and are reported on form 4797. so the MTM loss will first offset any other ordinary income you have. There is no $3000 cap on MTM account losses since they are ordinary. also don't forget that at the beginning of the year, unless this is your first year trading and had no positions in the MTM account at the beginning of the year, you have an unrealized gain or loss at the beginning of the year which must be taken into account to determine you current year profit or loss as determined under the MTM rules.
