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I've never bought a domain from someone else, what sort of price range are we talking about? I realize that it depends on the name itself, but are we talking $10, $100 or $1000 for two random words such as 'yellow scissors' or similar? I have absolutely no idea on what to offer them.
We also got a generic vital keyword phrase for just the reg fees after a domain trader let it go having been trying to flog it for 5,000 dollars for a good few years.
I would suggest you do not make an offer but ask what they want for it.
Usually the seller does not know what it is worth either... so pick a price you'd be happy to pay as a starting point and show them you are a serious buyer.
I think for a good domain $200 to $500 is fair. People will try to sell you total crap for $5,000, $10,000 or more. Some companies register everything they can think of, trying to monopolize certian segments of the market, then hope to find a couple of suckers.
For something like you mentioned, I'd pay no more than $200. But then again, if you're really going to spend years developing a site, get a domain that makes you happy even if it costs a little extra.
I'd tell them that you're offering X and that you need a decision by deadline Y and if they don't work with you on it, move on.