I haggle frequently when I buy something, it never hurts to try. I bought a large-screen TV about two years ago from the big blue box store, and haggled the price down by nearly $300 (and the TV was already on a clearance price). It didn't hurt to ask... I just asked the manager if the price was the best he could offer me. He said it wasn't, so I told him I'd found the same TV online for (insert price) and wondered if he could do something in that range... it turns out he could!
I've also recently haggled down the price on new tires for my car. They gave me an out the door price, and I'd already shopped around to know it was the best price in town. Just for the heck of it, I asked if they would take $50 off the total. As it happened, they would.
At flea markets, I always offer a few bucks less than I'd be willing to pay for an item. I'm not trying to be super-haggler, but if you don't ask for a lower price, you might be leaving money on the table. It's nerve-wracking at first, but once you get into the habit it kind of turns into a sport!