It really depends on how motivated the seller is, if you can take the time to know your seller and find out his / her reasons for selling, you are in a much stronger negoiating position than if you are just approaching someone on the fly with little or no information. General rule
of thumb is 10% discount for cash. Often times you can render a better deal than that, if you are well informed. Better make sure that you know the vessel you are trying to purchase
to obtain a good idea of the vessel's true worth. This approach works with an individual; not so much when dealing with a commercial
entity, they would rather sell it to you on time and collect the interest on the unpaid principle. Used to be back in the day the dealer would fall over themselves to sell for cash; I recently was kicking bike tires in a Harley shop and inquired about a price reduction for a cash purchase
and they told me they would rather finance it, this was on a new purchase, I don't know if a different rule
applies for a used bike. There is deifinitely a different dynamic when dealing with individuals.