Found this IMO really interessting article: "Hey day traders: Here are some tax strategies for you": http://www.marketwatch.com/story/tax-strategies-for-day-traders-2015-02-25 Published: Mar 4, 2016
Yeah, that covers the basics of tax issues for traders. I will point out that mark-to-market is a bad deal if you consistently make money since all your profits become ordinary income.
Unless you are "trading" for long-term capital gains (which doesn't fall within the IRS's definition of a "trader"), what is the difference between short-term capital gains and ordinary income?
If you elect MTM, it will take all your eventually long term positions and convert them to ordinary too unless you go out of your way to designate them as long term investments and not trading positions. There are a few other cases where MTM might be worse or more trouble if you are net profitable, but those get more obscure.