Looks good.

I personally prefer pixel art to have a black border around the outside, but that is just my opinion, this is fine as it is.

I think it looks quite impressive! I would perhaps suggest to add a sample animation somewhere in the page, to show potential buyers what quality to expect. From the screenshots I can tell the sprites look nice and detailed, but I wouldn’t know what to expect from them when animated. I can imagine a lot of viewers will ask questions like:

Is the animation smooth? Does it look realistic or cartoony? Will it fit the rest of my game (if I purchase it?)

Aesthetically, they look great! Loving the purple fire. The running animation does look slightly off to me; like it pauses just a bit too long between steps, but for a first attempt at pixel art I'd say you did really well.

Nicely done, really dig it!

You could also add some color changes to the flame (blue,red, green) to offer a greater variety to your customer.

I like the smooth color transition on the inner flame but at frame:

the edge of the flame (purple) got a kinda rough transition with no build up.

Hope i could help you!