Correct me if I'm wrong, but isn't the Airfix version of the Eagle the same as the version that was released here in the States under the MPC/Fundemension name? Additionally, the US release was repopped by AMT/Ertl in the late 90's as well.
I have one of the Product Enterprises Eagles, which also seems to be built in the same scale and a lot of the same parts, although with a much better tooling, decals and metal parts.
If it's basically the same model, but with different packaging, at least the packaging is unique. There's an Airfix kit on eBay that's supposedly a sealed, unused kit, priced at nearly 80 dollars. I found an original 70's MPC Eagle kit being sold via Amazon.com priced at 75.00. I even found an AMT repop being auctioned at 69.00 also.
Depending on what you could actually sell the kit for and the condition that it may be in, selling it for 30 dollars would be quite a bargain. It depends on whether you're pricing the kit to sell, or pricing it to get a profit from it.
Bryan