Pens.

I've always drawn with ball point pens. It never bothered me that I couldn't erase.  When I was little it was the bic clear handles. Everyone had them and they draw on anything. The line is medium size and slightly smooth. It was permanent but almost felt like a pencil, just don't put your line in too dark too early. They're cheap and I usually keep one with me at all times.
In college, a teacher of mine used Parker fine point refills to draw. I gave it a try and immediately loved it. Parkers can be super light and sensitive and they can get VERY black. I think they're about 3 dollars per refill but they'll last you many drawings. I love them for life drawing.
I've tried waterman fine points too. The line is alittle too dark for me.  Very good for fantasy artwork. I don't like them for life drawings.
With ballpoint pens it's always a good idea to have your shading lines going the same direction then cross hatching. As long as you keep it consistent. If you're going for sloppy keep it consistently sloppy. and your picture will look good. consistency and continuity makes for nice work. With some ball point pens, you can take a napkin and actually smudge the ink. Give it a try!