Its not an upgrade either way if you're buying a whole new machine.
As for my reasons, there's a few. First of all, particularly on higher priced items, a lot of the trouble with the price comes to having the full amount in the first place. I can't walk into an Apple Store £700 short but tell them "don't worry, I'll have the rest later".
Aside from that, actually selling the thing is no guarantee, especially not for £700. I remember going to buy a laptop on eBay over 3 years ago and I still see it up there now. It might not be as in demand as the MacBook would be, but the point holds. Even if it does sell, the question then is how long it takes before it does.
Based on that, I assume if I can only afford something if I sell something else, I can't actually afford it. There's just too many factors involved. If I can afford the thing and I sell the old thing after, great, that's a nice bonus.
