Here's my layman's advice, as someone who writes and publishes art tutorials. I write tutorials to help you learn a new technique or skill, so that you can become a better artist and have more tools in your creative arsenal.
If you are following my techniques and using YOUR OWN source images, or have added your own unique twist to make the artwork truly yours, then by all means, sell your art! I'm happy that I've taught you a new technique that you've found helpful, as that was my goal in writing the tutorial to begin with. Send me a link as I'd love to see what you've created. If you give me a shout-out in the description, that's a pleasant bonus that gives me the warm fuzzies.
However, if your art is an identical clone of mine, then use it as a learning experience but please do not sell prints of it. Instead, use the skills you learned in creating that first near-copy practice artwork and apply them to a new artwork that reflects your own artistic inputs as well - consider a new subject matter, a different source photo, etc. For example, this fall I'm publishing a tutorial on this fella:
If you sell a photo of your dog in a suit, wonderful! I'm proud of you! If you were selling my Uncle Jim's dog in my husband's suit, it would make me really uncomfortable...that would be a no-no. If that little voice in your head says "this feels unethical" it probably is.
Learn new skills and techniques and then add your own unique spin on your art -- something from your own artistic soul. Make it your own and you'll be fine!
