This is EXACTLY what day traders of stocks do. They have to calculate each buy and sell (which they do hundreds of times a day) and report it all at the end of the year.
The difference is your stock broker sends you documentation of your trades and cost basis (the good ones were already doing this years ago, I believe now there is some law requiring it.) E*Trade sends me a document and I give it to my accountant, it's really not that difficult or time intensive.