Making a shop that isn't a PokeMart

[AztecCroc]

Basically, I want to make a NPC where you talk to it and you can buy various items (for instance Apricorn balls or Mega Stones).

 

[HarmonyConcept]

You can edit the pbPokeMart script to include whatever item you want.

Alternatively what you can do is using some choice menu to sell your items and change gold consequently with an event command.

 

[Maruno]

You mean like a vending machine? There's an example of that in the Essentials maps - look where you'd expect to find a vending machine.

 

[The Evil Mewtwo]

Copy the Clerk event in the Pokemart and paste it anywhere you like.
You should try shops in Pokemon Centers.