Agreed, that's a project gun. And sorry, Charlie, I know this isn't what you want to hear, but the value really is dependent on a detailed description. Unless you know exactly what you've got, you're essentially selling a bunch of parts. For all we know, the internals on that are stripped out, or even missing entirely.
The spare parts, case and bag shown there are barely worth the postage. There are exceptions, but again they rely on knowing exactly what you're selling.
From what we (don't) know, I wouldn't pay three figures for the lot, including fees and postage. Someone else might pay that, and they may even get a real bargain. You don't ask, you don't get, but I'd be prepared for a lot of questions and potential buyers who mess you around and pull out. The more detailed pictures of markings you can provide, the better.
Depending how much time you want to put in, you could find a local airsoft site (e.g. on Facebook) and ask them if you can bring it all along to sell. Airsofters are actually a helpful bunch, you might find people on site willing to dive in and tell you what it is, and what they reckon it's worth. It's always more tempting to buy something if you can get your hands on it.
Best of luck.