you should respect other peoples artwork and not selling them as your own. i believe you didnt trace this time but its not your art now is it?
same goes for alot of stuff you SELL on redbubble. dude you cant make a study from other peoples art and then sell it.
maybe you are young and desperate but i guarantee you once you make some bucks people will come at you trying to sue the f out of you or even worse your reputation as an artist will be destroyed before you can really benefit from all the work you do.
other than that i think yuo are on the right track. doing studies of other peoples art is a very good thing to do for practice - just give credit to the original piece is all.
again - what i wrote is not to diminish the effort you put into this or the quality you are able to produce so dont get discouraged. just dont become shady because there is no need to.