Surely Buffy's problem isn't solved that easily, is it? What happens next?

No, it isn't. A more realistic scenario

"Damn it, Anya," said Kennedy. "I already wished you had a dress. Why do you want the one Jonathan gave me?"

"Because you still owe me, mystery girl," said Anya. "Besides, yours looks prettier than mine."

"Whatever," said Kennedy.

Just then, a cloud of black smoke that smelled of sulfur appeared in the center of the library, and then suddenly there was a black-haired figure in a black dress whose face looked much like Buffy's.

"Er, who the hell are you?" asked Kennedy.

"Buffy Summers, Justice Demon Local # 666," said the woman in black. "But you can call me Vengeance Angel Buffy. Or better yet -- ahem -- Dark Buffy."

"Er, why are you here?" asked Willow.

"I'll tell you in about five seconds," said Dark Buffy, gazing at the clock on the library wall. "Five... four... three... two... one!"

Just then a naked Buffy Summers ran into the library, closely pursued by a fully dressed Faith Lehane.

Buffy paused upon seeing Dark Buffy and asked, "What the hell are you doing here?"

Then she paused a moment and said, "It doesn't matter. I wish --"

"Hold that thought," said Dark Buffy.

And then she made the same magical gesture that Vengeance Angel Amy had used on Catherine Madison.

At which point, Buffy suddenly realized that she could not speak at all.