Even though I was in the collectible market of TBT mostly, I did however operate
one of the largest collectible shops in TBT history (managed over 150k+ TBT in assets before closing up due to priorities in my life shifting) and I have sold some things in the New Leaf market as well. My biggest advice for you is to come up with a service that no one else offers that you enjoy as well (even if it is a subcategory/class of an already existing service that no one seems to be paying attention to)!
Trust me, it is not worth starting a shop if it is no fun to operate. Additionally, try to offer qualities in your services that would make you hard to compete against (like offering reliable, friendly customer service or very good trading services for example). Depending on the volume of the market you are getting into, it also would be wise to be smart and creative on how to finance and grow your service/shop for long term operation (for my collectible shop, I had an investment program and Store Credit system that allowed my shop to grow while also helping others out in becoming richer in the process [essentially, a win win service for all parties involved]). I hope this advice helps you out a lot and I wish you the best of luck on your future shop!
Feel free to reach out to me anytime if you need a secondary opinion for a shop operation or to simply chat. o/
I know there are many other people on here that would be more than glad to offer advice as well (my very dear and sweet friend RedTropicalPeachyFish would be an absolute excellent person to talk to about the New Leaf market for example)!