To me it sounds like you're a very responsible 
artist!  As long as you clearly mark off that 
your work is fanwork, and that you put a lot more 
effort into it than just browsing what sold high 
on eBay and running off a copy of it, you are 
fine by me ^-^.  I think it's technically illegal 
to sell fancels directly for profit, but if this 
worries you a lot you can take commissions 
instead--it's my understanding that if a person 
hires you to paint a cel of someone else's 
character, then that's all right.  I wouldn't 
worry about it, though, since the law is not well-
enforced, if at all.  

Also, from what I have seen, the fact that the 
cel was done by a professional is not its major 
selling point; the quality of the image and the 
cel itself are what draw people to buy fancels.  
I have seen great work done by amateur painters, 
and terrible work done by professionally trained 
painters.  It all depends on the individual piece 
itself.  In your case, it seems like you are very 
skilled--the cel you posted is very nice. I'd 
like to see some of your original work, if you 
could post it.  I like Sesshoumaru and all, but 
it would be nice to see other characters ;).

Out of curiosity, which studio do you work for? I 
wonder if we might know any of your projects.