You are kidding me right?!? I said (in the past) I've done this and tripled my postion that began in May of last year! This has been done in my 401K. I don't have a tax obligation until I begin to draw on this, at the tax rate at that time. And funds I contribute are before tax earnings.
So if I pull off an 8% flip on one 10K block like I did last week, and another 20K that has been sold higher than current price waiting to purchase again next week in the 1.30's would be 15-20%...well...I'll continue to do this all day long if it's available.
Even 3% flips done 6-8 times in a month is nothing to sneeze at.