$300 is a little on the high side, as new P-95's can be had for that price. On the other hand, the decock models are no longer in production, so if that is what you want, it's not a bad buy. Great pistols, accurate, dependable, and easy to maintain.
Oh yeah, if it was a P95DCPR, I'd snatch it up for $300... My P89 was DC and I loved it... When I ordered my P95 from my FFL, I was under the impression that it was a DC model, BUT, it wasn't...but I took it anyways...
Buy it! Offer him a little less but if he doesnt accept still buy it. Figure it this way-they sell on Gun Broker all the time for around the $250 area +/- So if you pay $25 for shipping and another $25-35 for the transfer at an FFL youre at 300 anyways. And you can inspect the gun prior to purchase which is worth a few bucks imo. Buy it and you wont be sorry. The P95 is a great pistol for the money. Probably the best gun you can get for 300 bucks!
I think you really need to decide whether or not you absolutely want the DC only model. I've been looking on the net, and DC only models go around $400 on up, used. Ruger stopped production of the P95DC around 2007, and they are desirable.
If you decide on getting the newer safety/decock model, they can be had for $300 brand new in places like Bud's, and many others.