If it's a sell out gig you can try to return them to the promoter and you might get a refund, often there's a standby list of other punters. If it's not a sell out you're stuffed, but you shouldn't have expected to make a tidy profit in the first place. Folk who buy tickets without the intention of going is one of my pet hates.
Or you could try
http://www.concertlivewire.com, where you can buy/swap concert tickets.