How do you know if your sketching is worth selling? What materials are best used for quality?

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.

Pretty much whichever of your artworks sell are worth selling. There is no definition of "good enough" here, because art comes in many different media and styles. Every artist's practice is different.

I would recommend watercolor board or canvas paper. Do a search and you'll find lots of sources for both.

Art is worth selling in any form...if you like it, and someone else wants to buy it. Materials are irrelevant; draw with what you like, and make good reproduction scans if you want to sell giclee prints.

I would recommend inking your sketches on bristol board.