Very difficult to assess based on the information available. If the seller's assessment of 80% is based only on the cosmetics and mechanically it's perfect, $440 all in isn't bad; you'll likely be getting a good shooter with a fair amount of handling wear.
But 80% is pretty low for a nickel finish with no cracks or chips -- that's either a lot of handling wear, or the 80% applies not just to cosmetics but mechanics, too. If that's the case, I wouldn't buy the gun. Finish issues can be lived with free of charge; mechanical issues need to be corrected and that comes at a cost -- a cost that you could roll into the price of a different 6" Model 19 that is mechanically correct from the get-go and saves you hassle.
So key here is trying to determine that it's mechanically sound and in spec before you buy it.
As for words of wisdom for buying long distance, I'll borrow a post of mine from another thread:
1) Ask every question you want answered; ask explicitly and persist until you receive explicit answers. If questions receive anything other than a straight forward answer, walk.
2) Negotiate a fair return policy. This usually means a three day no-fire inspection period, with full refund provided you pay return shipping. It's personal choice, but if the seller won't offer a return policy, I walk.
3) Research and in general only buy from a seller with a demonstrable, successful history both as a forum member and seller to other members; look in feedback sections and PM previous purchasers for their advice.
4) Insist on FFL to FFL only.
5) Use a USPS money order; if you get burned, there is some Federal recourse.
6) Promptly communicate all updates and seek same from the seller, e.g., when money is sent, when firearm ships, etc.; all items sent or received should be done through reliable shippers (FedEx, UPS) and have insurance and tracking numbers.
7) If there are any issues whatsoever, communicate it privately to the seller and attempt to resolve it fairly and amicably before going public or seeking alternative redress.
8) When receiving the item, inspect carefully, determine if the deal is ultimately to your satisfaction, and if so, let the seller know immediately and, where possible, provide positive feedback.
