Just my honest opinion, but the first, second, and forth drawing does not look very good. It is very obvious that someone simply drew it, which anyone can do. The third drawing looks better than other, but again, anybody can create this kind of work. Therefore, I do not think these drawings would sell for anything. Even if they do, I would estimate around 10 robux.
I highly suggest to create and sell something that is difficult to produce.
Looks great, but not sure why you are costing your work a watermark at the bottom of your work for some mere text you can simply put on using a free editing tool such as Pixlr X. This service doesn’t even provide copy-protection, just a ‘watermark’!
I love the concepts you’ve painted, but maybe clean up the painting a tad bit! In the first one, I would change the black color to maybe a really dark blue, like midnight blue. Additionally, you could add a tad bit more white throughout the piece so that one section doesn’t stand out too much. With monochromatic pieces, you want to make sure they blend in a bit and don’t stand out too much. So maybe remove the white lining through the piece and blend the colors in a tad bit!
Overall, I think you just need to remove the huge sections of black and make it a darker shade of the color you’re using in the piece. It’s way too bold. If you’re going to use black, make it more transitional! Also, clean up your lines. The last one looks great, but it’s a bit messy.
I would pay maybe around 50 - 100 R$ for these! Keep up the awesome work, art isn’t easy.