It's a tricky one, I completely agree with bailey on points 1 and 2 just because they have bought the other extensions doesn't mean they are a dead cert. Go with what your happy with..
My reasoning is because of this:
Last year I got a £1000 offer on sedo, for a dictionary .com from the uae research showed they had just regged the .net/.org/.mobi and .info through a wildwest reseller.
I countered 5k expecting a dialogue they replyed the original offer was the max I replyed we would have a deal at £3000 and they cancelled the conversation.
The other domains are in the process of dropping.
I made the following mistakes,
-presuming anyone starting at 1k had more money
-presuming an because of the Arab country they would be expecting to haggle.
-thinking because they had bought the other extensions they would pay what a higher price.
-over valuing a domain which I would probably accept a grand for now.
Couple questions if the OP does not mind?
Is there anyway to find out more info regarding buyer ie almost agreeing to te contract to see the name (ie not pushing the button) I'm sure this used to work I reverse for buyin a domain when looking for owners addresses where there was a private Whois. (it no longer seems to)
Also has anyone had success at reducing the buyitnow to below the offered price
on a failed negotiation? I read on another forum that this would alert the original bidder.