Even though I can understand the sentiment of wanting an item you buy to maintain its resale value well, the idea of "selling it next month" voids any such argument.
I get a literal churning feeling in my stomach every time I see someone list a "brand new in box" item that has been removed from the box for pictures. unfocused's point about driving a car off the lot hits the nail on the head. Once you've opened the box, as a seller you should expect, and as a buyer you should probably demand, that the value decreases by about 15-20% immediately. It is at that very moment and no later that the item is "used", and resale value has everything to do with rate of depreciation, and almost nothing to do with its value after being removed from the packaging. The only, only thing for which that is not true is real estate, which is etymologically exactly why it has "real" directly in the name.
When a 5D1 is for sale, it's extremely common to hear potential buyers ask if it's had the mirror fix done. If the answer is yes, they are happy to buy it. NOBODY says "oh, it had the mirror fix, so it was a defective unit, and therefore I won't buy it".
A more extreme example is when people post MacBook Pros for sale on Craigslist, and the post says something like "2009 MBP 15", perfect condition, just needs a new LCD and hard drive and it will be good to go. $750". It sounds like a joke to any reasonable person, which is probably why I see the same posting still up a month later.
I know this was an incredibly roundabout way to say it, but buy the camera you want for yourself, use it well, and when you're ready to sell it, consider what you get for it a bonus. You'll be a lot happier than if you consider whatever you fail to recoup a loss.