Where to learn to make npc seller by using lua?

Is there anywhere that I can find a tutorial about making npc seller? Like, a players able buy something from npc seller, I saw some of them on some server, so I thought I would make myself for my server.

Thank you if your helping, and if you saying something that are useless for this thread, don’t post it, please…

just make a snpc and use Ent:use to open your derma

I couldn’t find how to make snpc, so do you know where the making snpc tutorial is at? If you do then can you please kindly give me the link for it, thanks.