How do you judge cotton quality?

I would like to buy and sell cotton clothing. Superior knowlegde of cotton quality would help my negotiate for better price.

One thing I realized when looking at different wholesalers in my area, not all cotton is of same quality. I would like to improve my knowledge on grading cotton quality. With good quality cotton you can usually tell right away by look and touch of material, but how good is it? what is it actaully worth?

Does anyone have a good website? I googled and have yet to find anything very helpful... i'm going to try a few re-wordings.

Is there somewhere I could purchase a booklet of cotton samples? If I had samples from low grade to top quality, I could compare against finished product by touch, until I get a sense for actual value.

Are there quick test you can perfrom on material to see if its dye is going to run?

I need to haggle for price, and good knowledge of actually value is needed.