Sure, there are a thousand good reasons for someone selling a great gun. The problem is, you don't know the reason, so there's a bit more risk than buying a new gun. Personally, I just don't like buying a used gun from someone I don't know or trust. Just my preference.
There is risk in buying a new gun also, especially if you require the kind of accuracy I do. I try to minimize that risk by reading forums like this to get a consensus on the typical accuracy of a certain gun model. This has worked well for me. That is why I currently own Tikkas, Sakos, Savages, CZs, and Anschutz rifles. I have not been impressed with the accuracy of Ruger rifles in general, although the rifle you are considering may be their most accurate. Only an accurate rifle is interesting to me. Not having to work hard to find an accurate load make me like a rifle a LOT more. My favorite guns shoot just about everything well.
I've had experience with three of the exact same model rifle you are considering, so thought I'd share that experience. Me and my three friends where all considering buying that Ruger at the same time. I researched it to death on the forums and with the Varmint Hunters Association and concluded that I could not expect the accuracy I wanted out of it, and that the heavy-barreled Savage varmint rifles were more accurate. Three of my friends ended up buying it anyway. I ended up buying the Savage 112 BVSS instead. Our results mirrored that of the other owners on the forums. My Savage was the most accurate; their Rugers had a nicer shaped stock, were less club-like, prettier and nicer to handle. To me trying to hit a prairie dog at 300 yards, accuracy was everything, and it resulted in me missing less shots than my friends. The difference between 3 inch groups at 300 yards and 1-1/2-inch groups at 300 yards is huge when trying to hit a 4-inch wide prairie dog.
With varmint rifles, you also have to wonder if the barrel is shot out after 15 or 20 years. With a .223 like the gun you are considering, this is usually not too much of an issue. With a 22-250 or .220 Swift, I would be concerned. I put more than 30,000 rounds through that Savage .223 in 15 years before I sold it recently. I'm not sure I'd want a .220 Swift with that many rounds through it, especially since when shooting prairie dogs, you can shoot more than 100 rounds per hour, and even thick barrels get WAY too hot to touch, even in .223. I also didn't want to stop to clean it in the middle of a 500-round day. If the original owner lived in Wyoming, I'd be more worried than if he lived in Connecticut.
However, if you like the gun, take the chance. That's a good price, and you can do much worse.