Odd that a seller with so much feedback (1100) and a positive rating of 99.5% would suddenly take to scamming. £400 GBP is a lot of money for even the coolest new Blackberry. Plus, if it's a pre-production unit there could be trouble obtaining support should it ever become necessary.
Looks like he's a legit seller and has sold phones before.
I was offered well over 5 grand f I could get my hands on the first pearl (8100)I saw. It didnt release for another 5 or 6 months. But I got to play with it because I a guy I built a house for works for RIM. I couldn't ever catch him at home to ask him about the deal. I stood to make a nice chunk of change by just being the middle man. So people who have liquid cash will pay anything for a device or gadget that no one else has.
keep in mind that even if this device is legit, its internals may/will vary from the final retail version, even though indications that its an engineer/pre-production markings are no where to be found.