There’s nothing wrong with promoting to sellers. If you are spending your time and effort, however, promoting at sites where the members are largely sellers themselves, then consider this….
….Are you selling media, tools and supplies for artists and crafters? Marketing to sellers in this case is a good practice. If you are selling handmade/handcrafted products, why are you spending the majority of your time, and maybe some dollars, marketing to others who sell handmades as well?
It can be like marketing your eggs in the chicken coop. Why would the other chickens buy any from you when they can lay their own at any time?
If you want to market your products to other sellers, be sure that there are enough of them, where you are spending your time, who use creative techniques aside from yours. Someone may buy what you are offering because they can’t make it themselves.