Used Stubhub for years and they have terrific customer support...
Instant downloads have always worked fine.
Back when State went to Little Rock for the NCAA basketball regionals, I had bought a book of tickets for the regional in Birmingham the previous spring. So I bought Stubhub tickets for Little Rock and used Stubhub to sell my Bham tix. I sold Thursday's tickets to someone in Alabama, Friday's tickets to someone in Tennessee, then accidentally sent the Thursday AND Friday tickets to the person in Alabama. I panicked and called Stubhub. They contacted the person in Alabama to send the Friday tickets to the person in Tennessee. They were going to guarantee tickets to the buyer in Tennessee if my tix didn't get there in time.
I know Stubhub makes a lot of money from the sellers and buyers, but the ease of their website and excellent service have made me a loyal customer.