Not sure if this would work these days, since this could get out of hand, but you could probably hang out around the "will call" area at Staples just before the game. I've noticed that people who get their tickets there sometimes have extras. When I attended games earlier in the decade, I occasionally sold a ticket or pair of tickets this way... and not for profit. (Haven't been back in a few years).
Special guests can usually obtain up to 4 tickets at comp or absurdly low prices. And so the tendency is to buy four tickets even if you aren't sure you'll use all of them. Then... Somebody in the party doesn't show... or perhaps the buyer ends up using only using 2 or 3 of their 4 comp tickets.
Stand near will call and DISCREETLY ask if anybody has extra tix. Dress nicely, look sane, have cash handy, be the sort of person who might be fun to sit next to. This is clearly a last ditch effort... but it still might succeed at times if you can pull it off.
Dr. Dr. Jeckyl