It's a question that comes up time and time again and it's a subject that people seem to have a million different opinions about.
You have to look at things from both the buyer and the sellers point of view in my opinion, a lot of sellers are motivated to sell and even if they realise the offer on the table may cost them a few £ in the long run it ultimately means they get paid quicker - whether or not buyers rely on that factor or they are just trying their luck with a cheeky offer the seller is the one that ultimately decides if the offer is good enough to accept.
If I were a seller without a clue and shortly after listing something got a cash offer to end early the alarm bells would be ringing. I would then research that item as much as I could before I got back to them. It's not all that hard to use google or even ebay to find out a bit about what you are selling and if you don't bother before you list it you should at least try to if you suddenly find yourself being offered money to end early.
Now for the buyers...IMO being a "nice guy" buyer on ebay is like being a Dolphin in a sea full of Sharks, you might look similar but you are a totally different kind of animal. The majority of people who buy on ebay are looking to buy cheap and resell or they are just looking for a bargain for a variety of reasons (not much cash + collecting on a budget/plain old cheapskate etc) which in turn means they are always going to try their luck and end an auction early.
Can you blame people for trying? I mean as much as it sucks to watch an auction get pulled when you had every intention of bidding/winning there isn't actually a penalty system for sellers if they end their listings early.
I think a better question (because this is so widespread and looking at the votes..accepted) is this: If you are going to try and end an auction early do you lowball the seller hoping for a bargain or do you actually pay a fair market value?
I know people that do not want to risk losing out at auction and therefore they put their best offer on the plate and see if the seller is tempted to make a quick sale at a good price. Maybe you forget to bid on that once in a lifetime item..your sniper tool doesn't load and no bid is placed..someone gets drunk and overpays leaving you in 2nd place..anything is possible and asking a seller to end early doesn't always mean you are trying it on.
FWIW- I have asked sellers in the past to end early on ebay if the item was something I couldn't let go or I didn't want to take the chance of losing..however my success rate with that is pretty low lol
Don't forget you can still be a nice guy and play the game. Saving money isn't always the ultimate goal of ending early (however I can't knock anyone who tries to save a few pennies as long as it's not at the expense of every seller they deal with..you have to be fair to some extent)
Joe