Hello there, KIRIL!
I'm assuming you want the following scenario to occur:
A merchant travels to five or six locations in a map in a circular motion, periodically stopping at each to sell his wares, without being interrupted. A hero can purchase items while the merchant is moving, as well.
This is fairly simple to set up.
I've created a map that shows how you can do this. Please check this post's attachments to download it.
I'd suggest checking the map both in-game (to see how it works live) as well as having a look at the object and trigger editors to see what I've done. In the trigger editor, I've included explanations/descriptions on each of the triggers to help you understand what I did, and why I did what I did.
If you have any further questions, please feel free either to reply to me in this thread, or to send me a PM. I hope this helps!