If a "borrowed" design improves upon it predecessor, then it essentially becomes a new product and can not be considered stealing.
However, I see items coming from China that are plain ripoffs. In my field of ultrasonic cleaners, China is selling (40khz) ultrasonic cleaners at an impossibly low price - in my opinion, it is an unfair tactic to shut out American (and other countries) manufacturers - to essentially put them out of business - then China will inflate the price to recover their losses.
Never build before you have a confirmed buyer, I believe needs clarification: Never build and stockpile merchandise before you have a market. I did this for my first invention- an emf detector with a frequency readout. I sold two in five years, but have inventory to make 100.
Innovate to meet customer demand - I got (and still get) messages from customers about what they want and need. Customer feedback is why I added 80khz Ultrasonic systems and discontinued the 40khz systems. I am also consulting on some other projects.
Optimize for speed and small runs! Buy only the supplies you need to meet demand. I made the mistake of over-ordering parts in anticipation of sales, only to have my bank account drain. Now, I buy parts as I need them, and just enough to complete those orders. Keeps more money in the bank.