Nathan Tobey went to a Phish concert hoping to enjoy the medley of song improvisations and laid-back vibes that have drawn followers to the band for more than three decades. Instead, he found himself in the middle of a vicious battle for real estate.

When the gates at Virginia’s Hampton Coliseum finally opened after an hourslong wait, Mr. Tobey rushed to stake out a choice seat on the first row of the balcony. Seconds before he got there, another fan swooped in and covered the entire row with a giant tarp, grabbing all eight...