The quality is quite professional. I think this level of work is indeed worthy of selling.
Looking at fire sprite animation, perhaps you could provide smaller “stride” variant(s), so that developers could synchronize sprite leg position with movement speed without “sliding” effect. Current versions supposes very fast movement.
You may think of offering additional optional layers, such as shadow of items in hand, or maybe a few pre-made palettes.
Here is (very exaggerated) example of what I am suggesting: https://kingbell.itch.io/