Even estibot says you can not use them for any meaningful determination of real value. There are too many personal factors involved with domain name sales and purchases to really know how much a buyer will pay and how low a seller will go.
In your case you are in the worst possible position as a buyer. You really want the domain(s) and the seller doesn't really want to sell. Depending on what kind of trader the seller is they could really squeeze you on this.
In this case since the seller doesn't really want to sell, you should ask them to set a price. The reason I would do this is because you need to see where they are at to start with. If they say $100, then you could get them down to $40 maybe. If they say $1,000 then they may not go lower than $800.
If you offer $100 they may go for it, of they may figure you are offering 25%-50% of what you will really pay. If you offer $20 they may decide you are never going to come up to the price they want and try to withdraw. If you say $200 and that is all you can pay, they may think you will still offer more if they hold out, or they may jump at that offer.
So, in my opinion, you should tell them to set a price and encourage them to make it fairly high. Sounds stupid, right? Well, if you do that it will put them off their guard. You make it easier for them to name a price that will make them quite happy. Then you know what their best hope is. If they say $300, then you know you should be able to get them down to $100-$200, since if $300 is the price of ultimate joy, $200 should be the price of happiness, and $100 may be the point of satisfaction.
If they say $1,000 or more, then you need to do some work to get them down to what you are willing to pay (say $200). At first this may seem impossible, and it may be but perhaps not. You can attack their price with examples of what the other TLDs have sold for or domains like this, or domains that are much better than this. You can talk about the lack of keywords, of the fact the domain has no PR and no backlinks. You can also mix in some positive things about the domain so you don't just sound like you are trying to pull them down. Try to state all the facts and just don't play up the positives too much.
If you have to offer a price, do some research and get as much information as you can as stated above when the price is too high. They wany $1,000 and you offer $200 and explain in as much detail as possible why. DON'T make anything up, just present all the facts and don't say too much that's positive.
Another option for buys like this is to try and find a name that's EVEN BETTER for what they say they want to use the domain for. If you can reg of find an even better domain then you may be able to make a trade or trade plus some cash. Heck, they might even give you some cash if you find a domain they really like...!
Every transaction is different. Your best advantage is to not be that interested in the selling or buying, and have as much information about the domain as possible.