Also, try not to infuriate the seller. I test drive bikes pretty hard. Sometimes hard enough that when I come back, it's pretty much sold because they're angry at me and worried I damaged the thing. The guy selling his son's 24" Diamondback BMX as a "kid's bike " was definitely not pleased. The guys helping him move were more impressed with me jumping down a stair set and hopping up curbs than he's was, they were almost laughing at the look on his face. If he weren't moving that day, I doubt I would have gotten the bike, just out of spite. His asking price was good enough, and combined with his mood, I didn't bother to dicker.
When I test rode some Hardrocks, I did it out of anger; he listed them as Rockhoppers so I let him watch as I hit them HARD! He had about 6 he was selling for a summer camp, and I bashed every one. Ditches, more stairs, jumps and bumps... and I fell in love, bought 2 of them. Boy, I showed him! I think I might have gotten $25 off each because of the package deal.