Do I need to purchase a license?
Let's say I built a software using QT and now I want to sell it, do I need to purchase QT's license before?
If so, which license should I buy assuming I only want the option to sell software I built once (ie I do not use QT regularly, for a specific project I used QT)?
Plus, assuming I buy a license, is there another benefit I get besides the legal right to sell the software I create? Are there any new features added? Is there any protection added to my software? Is there anything other than the legal right?)
QT's official website is not clear on this issue ...
Most when not all of your questions are better addressed to Qt sales. They can give you detailed answers to what is most suitable and to your rights when you have purchased a license.
This is basically an open source user forum. Even though they might have had similar questions before, they decided to go for GPL or LGPL.
The forum is not really monitored by Qt and especially their sales department.
However, be aware that certain parts of Qt are GPL not LGPL (also that can happen by programming language, e.g. Python/PyQt), and there is also the issue if you statically link. Then there is the thorny issue of where the Qt source code is "available" from. So you also need to look at Qt's licensing carefully.
I personally believe, that QT's official website is purposely vague on that topic. I contacted the sales team multiple times on behalf of different companies and the answer I got was always an interpretation of:
"To be sure you, you better by a license"
The Qt Companyis a company after all.
However as long as you yourself are not a certified lawyer, I would suggest contacting one to handle the issue for you.
If you have to buy a license, the cost of the lawyer won't change much in your expenses. If you don't need a license, It's a minor cost to bear, but at least you can be legally sure.