I would argue that cheapest doesn't have to be much less safe than the safest. It often is, but the right carefulness off-sets this.

Before buying my N900 second-hand off eBay (I would've gone with Dell had I known of the coupons they regularly have at the time though) I learned what distinctive features could be determined from just the pictures of the phone, that could determined if it's fake or not. I mainly used the letters: The N900's J does a 90 degree turn. So if you start from the top it goes down and then turns left. The fake's keyboard letters have a font that make the J start going down, but then the curled part turns all the way around and up. The Q in the N900 has the little stick start from the circle and go out; the fake's Q intersects the circle of the Q. The real's M has the bottom of the middle part of the M end higher than the two outside ends; the fake has the M's middle part reach all the way down to the same height as the other parts of it are.

I also immediately discounted any N900 that was being sold without obviously unique pictures. If it used the stock images that you'd see on adds or the sites advertising it or other official stores' product images, I wasn't willing to risk them. (I also made sure the 'unique' pictures didn't come up for other N900s sold by different sellers)

That said, an enterprising scammer could still scam you, but it's harder at that point. If you can get Dell's offer to work for you in the UK, do it. So far, the cheapest, even cheaper than most used ones, are the dell ones when you apply coupon. Even a 50$ one would save you enough money to make it cheaper than most final selling prices on eBay, and last I checked dell's normal price for them was cheaper than Amazon's or Nokia's.