There exists an event comment which is turned into the basic scripts necessary for selling an item. The wiki has further details, but the basic usage is like this:
@>Text: \GHi! Would you like a SlowpokeTail for only $200?
@>Show Choices: Yes, No
: When [Yes]
: When [No]
@>Text: \GOh, that's too bad.
: Branch End
It's useful for vending machines and NPCs like this, and saves you a bit of hassle getting them set up. To make a selling one-time-only, change the event's Self Switch when the item is obtained, as described above.