I see nothing unethical about that, but there are legal considerations. (Disclaimer: I am not a lawyer and this is not legal advice.)
If you’re using existing code, they may still have a stake in the IP. You may need to make sure they either license it or turn over ownership to you. In the latter case you may want to set up an official bill of sale and pay them something for it, even if it’s only $1 each.
AFAIK there’s no legal ownership of the idea if it hasn’t been patented. But your options may also be restricted if you signed a contract that includes a non-compete clause.
If you’re using existing code, they may still have a stake in the IP. You may need to make sure they either license it or turn over ownership to you. In the latter case you may want to set up an official bill of sale and pay them something for it, even if it’s only $1 each.
AFAIK there’s no legal ownership of the idea if it hasn’t been patented. But your options may also be restricted if you signed a contract that includes a non-compete clause.