I worked at a dealership for a while so I got to see how things work from the inside. There are tons of different factors. On new cars the auto makers offer various incentives or specials to dealers that might allow the sales person to go lower on Thursday than they would on Monday. It also helps if you're flexible. If you don't mind waiting a week to pickup the car or can take it in a different color you're making the sales person's life easier and they'll usually give you a break. If you're really picky and demanding they really don't care if you go to another dealership so they won't offer you much of a deal.
For used cars the dealers are making money on trade-ins even if they let it go far below the posted price. Any used car they sell off the lot is going to be a bigger profit than cars going to auction. A lot of times you can simply look at the dealer's lot and see how crowded it is. If it's packed with lots of used cars there's a great chance you can walk in and say I will pay $X and get the car. They just want to get it off the lot to make room for better inventory.
My advice, if you're looking for a used car, is to find a dealer that doesn't deal with a huge volume of used cars. They're often selling their trade-ins at a good value because they don't have the volume to make the auction process work in their favor. They'd much rather just sell them off the lot.
For used cars the dealers are making money on trade-ins even if they let it go far below the posted price. Any used car they sell off the lot is going to be a bigger profit than cars going to auction. A lot of times you can simply look at the dealer's lot and see how crowded it is. If it's packed with lots of used cars there's a great chance you can walk in and say I will pay $X and get the car. They just want to get it off the lot to make room for better inventory.
My advice, if you're looking for a used car, is to find a dealer that doesn't deal with a huge volume of used cars. They're often selling their trade-ins at a good value because they don't have the volume to make the auction process work in their favor. They'd much rather just sell them off the lot.