One thing I always do is make sure the thread I use for piecing/quilting is 100% cotton. That's because I use only 100% cotton fabric. I've always heard and read if you use poly thread on cotton fabric the fabric will wear out before the thread leaving holes rather than something than can be restitched. Again, this is just my preference. I also always heard NEVER to use c&c because it was so linty and short staple it was unreliable. I had used c&c for years making garments and crafts so I didn't understand but I obeyed when I started quilting. Then someone gave me several large spools of the c&c star thread so I started using it for quilting. Loved it. Still do. I have started using the presencia 60/3 for all my piecing. I really like it best. I tried several others but it seems to work best for me. The thread for quilting, I still use the star for some but I've also started using the connecting threads thread also. Some swear by aurifil but it, me, and my machine didn't play well together. As many have said it's all what works for you and your machine and what gives you the result you want.
I am extremely frugal and usually always wait for sales when I buy fabric but even I think you should buy the best you can afford. Not necessarily the most expensive, but best quality. As many have said, you have to feel the fabric to get, well, a good feel for it.

I will only quilt with 100% cotton thread for piecing and quilting. If you use the same fibre as your quilt fabric it is much less stressful on the seams. This is information that I learned when I took textile classes when I became a seamstress. As an example many quilts in the 1970s were made with polyester or poly/cotton blend thread. The polyester is a much stronger fibre than cotton and over time "cut" the seams.

Why do you want to sell your work? Are you wanting to sell at craft fairs? If so, go see what prices people are paying. I very strongly doubt you can make a profit with $9 a yard fabrics and top quality thread. The few people I know who sell at craft fairs do it purely for the fun of it, there is very little profit made. If you want people to pay you to make quilts for them, do some searches on this board to read all the problems to be had doing this. I have yet to read of anyone who felt it was worth their time to sell quilts. You are lucky to even cover the cost of supplies, much less your time and talent.