I think it comes down to the condition of the gun, your desire to have such a gun, and how long and hard you want to look to find another one that is in comparable condition. You might be able to save $50 or even $100, but then again you might not find another one at all. They are becoming more scarce and more expensive as time goes on. Even if you have to pay a little more now, you probably won't lose a penny if you keep it for a while. SOMEBODY will probably pay $550 for it now, especially with two extra mags. Not sure what the holster is, but it could be worth a few bucks too.
If you found a nice 3913 at a local gun store, you'll probably have to pay sales tax on it...so figure an extra $30 or so. The mags are easily worth $30 or so apiece. If you buy it online, figure in the FFL transfer fee and shipping. Bottom line: You could easily rack up more than $550, and not have the convenience or peace of mind of seeing and having the transaction done right now. Not to mention being able to buy it off paper, possibly.
I'd buy it if it was nice and if I wanted another one. I'd pull some cash out and try to make the deal. It looks like a nice one.