How do I make NPC that runs an in-game store

Hello friends!

I'm new to the software (only a few weeks) and I really enjoy it, I produce a simple platform game (of course - this is after all a first attempt) and I am interested in creating a store within the game.

That is, the player arrives at the NPC, a text box of "Want to see what I have to sell?" Then with approval a shop window opens where he can buy things like more life points or fireballs to throw.

I searched the internet for some guides to do such a thing but did not find - maybe you can help me please? Or refer me to appropriate instructors?

I appreciate it very very much

construct.net/en/tutorials/series-in-game-shop-117

This is a Construct 2 tutorial but the concepts are still the same.

Thank you! But since his files are non-existent (because I guess it's pretty old) it's impossible to see what the end result is and it's very confusing.

Maybe there is a guide or other explanation for the subject? Thank you very much!