If you wish to use materials of high quality for the purpose of making your art more "saleable", you will want acid-free, neutral PH paper to ensure your art doesn't succumb to age. However, that's an expensive avenue for sketches. By sketching, do you mean a finished drawing or illustration, or a quick, loosely drawn image lacking details? I suppose I'm trying to say that the quality of your materials might appropriately reflect your style or intent.
As for determining what is worth selling, I echo the sentiments of the other respondents here-- it's worth selling if someone wants to buy it. You can strive to make anything you do worthy of being desired by someone else, but you can't decide what someone will want. All you can do is put your best effort into everything you do. If you sell some art, find out what the buyer liked about it. That will give you some clues.