Anyone who has a backstage pass will have it for two reasons -
they arew friends of someone in the band or connected to the band, and wou;dn't sell their pass to a stranger
they are there for professional reasons - same as above.
As a music writer, I get backstage passes to gigs, but if i started flogging them to strangers, my access would soon stop!
Basically, you have next to no chance of getting a pass as a fan - the bigger the band and the venue, the more tightly controlled the security - strangers are spotted very quickly, and passes are checked.
Sorry to be the bearer of bad news, hope you enjoy the show.