I purchased two premium domains in my life. Buying it at a discount takes patience.
What worked for me is a combination of things.
- mention that you have narrowed your search to a few domains, to convey that you have options;
- if the asking price is too high, inform the seller that it is outside of your budget currently, but not by much (if that's in fact true);
- if he doesn't budge on price, tell him you'll have to think about it;
- if you really want the name, see if any other gTLD is available with same keyword that might work for you. Buy it and setup, if possible, a landing page;
- approach the seller again saying that you have purchased the same keyword domain in another extension that theoretically would work as well, but you'd prefer the .com if possible at an X price. When someone sets a website running with the same keyword, it slightly diminishes domain value, in my perspective. Only because anyone else wanting to get into the same name will now know he won't be able to protect the name by buying the secondary gTLD extension.
Both times I was able to cut a better price. The first time around 40%, the second about 15%.