Yes it’s easy, but you must have cash and be 100% decisive as well as being able to assure yourself the car it OK in that time.
A dealer wants his car on the forecourt for the minimum time so will always want to sell that day, saying you’ll think about it is a dealers nightmare.
If the car is prepped there is no reason not to.
I’ve done it 3 times, and despite what others have said it normally leads to the biggest discount. No substitute for telling a dealer it’s cash today if he shifts himself – last car I bought was a 20% discount, but it was month end and he wanted a sale. Car supermarkets and run of the mill cars there will be less of a margin.
Easiest ones are when they have a car that is not normally their thing – normally a trade in. If it’s say a VW dealer and you’re after a golf then it’s not out of place on the forecourt, so they can afford to sit on it as they know another buyer will be along. A focus at a BMW dealer – much more keen to get shot of it.
You can tell in the first minute or so if they are up for it.