I cannot answer the question about WordPress etc. as I have never used them, but the key point is that I suspect that you need a development environment that links to an SQL back-end. You can avoid that if you are dealing with a small number of high priced items, like houses or cars, but for everyday purchases, you need volume, and volume means a database. So if the development tool does no have that, it is no use for the app you are thinking of.
Database design is a task not to be taken lightly. And I suspect that your manual input idea is wholly unrealistic. Do you realize how many thousands of different items the average general store sells? So I suggest that you need to progress in stages, as I am doing:
a) Learn HTML and CSS (or whatever front-end development package will fit the bill).
b) Start to integrate PHP, without SQL, to learn how to handle variables and to make the web pages more flexible, and easier to maintain.
c) Integrate an SQL back-end to handle the volume of items you need.
I'm four years down the line, and I currently at stage b). I have yet to implement an SQL back-end, even though my background as a programmer is in 3GL database design! In my experience, there is that much to learn about the front-end before you worry about the back end! Of course, a web design package like WordPress will speed up the initial sage by shielding you from much of the complexity, but if it does not have the ability to integrate with SQL, then it could be a blind alley for you.