It's a difficult call for a couple of reasons:
New sigmas (and now SDVEs) sell for @ 300 new with 2 mags. Sigmas have always had a low resale value. He only has one magazine which further deflates the value. However, the unknown here is the holster??? Is it a valuable one? Assuming the gun is in excellent shape (with one mag), the gun would probably draw $200 in resale. Unless that holster were gold plated or something, I'd offer $225, maybe a few dollars more. If you can get your hands on a new SD for $300, I'd go that route.
Again, the value of that holster is the wild card here.
All that said, I have always said that the sigma was the best $300 I ever spent. Perfect performance and splendid ergos.