A scalper was selling tickets near the concert venue.
Buying tickets from a scalper can be illegal.
Standard spelling using katakana for the slang element ダフ and kanji 屋 for 'dealer'.
Hiragana variant; less common but still encountered.
From reversing the syllables of ふだ (札, ticket) to だふ, plus 屋 (dealer). The exact origin is uncertain, but it is a well-known piece of Japanese slang.