@JustTakeAction said it best, just start building.
Here is what you need to know about how to do this:
1) Buy a domain, preferably related to what you are selling.
2) Start selling from that domain.
3) Treat it like a business that has to succeed or you do not eat.
4) Once the business is doing well, perhaps with 2 years of history, start looking for buyers.
The beauty of the wild wild web is you can do this over and over, with multiple projects running simultaneously. The more you run, the more refined, hopefully, you will become, which will increase your efficiency and profit margin.
Most people just take the dive and start somewhere, anywhere.
Yes, you can use WP, Wix, ChatGPT. Use every tool you ethically can to succeed.
Also, I know your topic mentions Flippa, and you mention them again in your post, but consider that there are other platforms and ways to sell then just Flippa. https://www.blackhatworld.com/seo/flippa-alternatives.1600704/post-17735615
Do not overthink this. Just pick a topic and run with it. If it fails, you will probably be out less than $100 and some time. Learn from everything you do, and improve. That is how you get experience.
Good luck, and feel free to post your Journey here in BHW.