Are sites like Stub Hub okay to buy concert tickets?

I'm an Aussie so please forgive my ignorance. We're visiting Portland and trying to buy tickets to the Foo Fighters show. The link from their site (Ticketmaster) says tickets are sold out but I have found them on sites like Stub Hub and others. It seems like these guys just buy them a bunch of tickets then sell them at double the price (which seems very illegal) but is it okay to buy tickets from them? We know we'll be overpaying but we really just want to see the Foo Fightera. Thanks!