Maybe it's just me, but I try to pay with exact cash when negotiating a price. Seems like bad form to look at a $40 item, negotiate down to $30, then hand the seller two $20 bills.
Another benefit of not getting change is that it protects you from taking counterfeit currency. I don't think that TCA members would knowingly pass fake bills, but some of it can be pretty convincing, and it's becoming a widespread problem even with smaller bills.
Many members (and most dealers) will also take checks.
In the dealer halls, I always ask if there's a break for cash or check. If not, I pay with plastic and get the points. But I've found that most dealers will discount to at least cover the tax if paying cash.
