If you are referring to the 17' Perception and if it is in nice shape, it probably is worth the $500 (maybe even more) the fellow wants for it. However, it never hurts to make an offer.
Yes, the boat would definitely give you some idea of what a sea kayak is like. Keep in mind that not all sea kayaks feel alike, nor perform alike.
There are at least a couple of things that I would look at very carefully on any used (especially poly) kayak. Turn the boat upside down and sight down the keel. Is the boat straight from end to end. The keel (bottom) will naturally have some curve (rocker) to it as opposed to being flat, but if a string were stretched from one end to the other, are both ends and the center lined up straight. This is not always easy to determine, because of the complex shape of sea kayak bottoms, but if you have a keen eye and study it for a while, you should be able to tell.
The other thing I would look at is how rigid the boat is. Put the kayak on the ground, or floor; go to the stern and slowly lift the stern with the toggle, or handle amd watch to see how much the boat bends as you lift. Most of the flex, if it has much, will be in the area behind the cockpit. If the boat flexes very much, I would be skeptical of it. It would be great if it doesn't flex at all, but a lot of poly boats flex a little. You could go to a dealer and try this on a few boats to get some idea of what might be normal. If possible, check a Valley Aquanaut Club; they flex a lot.