Some good answers and insights here, so thanks! I did start of being very polite in my responses to offers, but that was until a buyer was very rude for me declining her best offer! But I think the people that take a break and come back to the offer later perhaps is a good way to go!

I have most of my items on Best Offer listings partly because it saves listing fees, and as others state it allows for some flexibility on margins and how recent sales have been going! if it has been a slow week or so, I am perhaps more willing to take less, if I have had some good sales then I am in no hurry to take offers. Also a lot of my items cost $20-30 to ship due to weight and insurance, so Best offers allow me to take the delivery suburb in to account as well as my margins etc..Best offers basically allow for greater flexibility  because you don't rule out those who want to pay a bit less and you can adjust for many factors, including as others mention those buying more than one item, at least that is how it works for me!