I recently bought a domain name from a premium domain seller and I asked the same question. The original post can be found here: http://news.ycombinator.com/item?id=4278380

Don't make the first offer. They'll probably give you a price higher than what you offer and they just want to see how much money you're willing to spend. You contacted them about buying some of their domains. Ask them how much they are willing to accept for the domains.

I was able to cut the price of the original quote almost in half when buying my domain because they gave me the price first and I responded with a final offer at a much lower price and said that's all I had in my budget for the domain name. It was a polite "take it or leave it" offer and they took it.