At the start of the thread, it would not have been assumed that the OP would keep retail packaging. Now that it has been mentioned, it's probably safe to assume so. In either case it's used. There's no need to disclose that it was purchased refurbished. I would personally disclose if I knew something was wrong with it. If someone asked if it was from the refurbished store, I would tell them. The way the OP is written suggests that the refurbished store carries units that are somehow tainted. The forum tends to hyper-analyze things such as backlight bleed, image persistence, and variation in color temperature relative to other devices. No one has ever brought up complaints with the quality of refurbished vs. new.
Where apple differs here is many manufacturers clearly stamp refurbished items. Personally I buy refurbished apple stuff for myself or family with no hesitation over retail items and do so whenever possible. As everyone else states the quality of the apple refurbished items is impeccable.
In the case of selling, ethically, it would have to be mentioned if asked.
What you haven't mentioned is how much of a difference retail packaging made in any given situation. Refurbished imacs of the current generation are $200-300 lower than new assuming you're just looking at the Apple Store and not at the student discount rates. Were you really able to eclipse those amounts simply due to retail packaging? I have no idea how much more. I assume it was significant enough to mention it in the first place, although I do think they were suckers.
I wasn't looking to write a detailed essay on the subject, nor did i state apple goods were what I had been selling. For about 10 years through school I bought items at flea markets and yardsales and resold either at the flea market or via want ad/craigslist type methods. Primarily electronics goods such as video games due to the margins being high for the size of the items. But really anything that a profit could be turned on was fair game.
Off hand for a number to throw out lets say cosmetics and packaging were good for 10-25% difference in initial asking price. This number would vary due to the item in question, time of year, stock on hand, demand for the item, etc. The % relative to price typically worked on a declining scale. I.e. cheaper items benefited more than more expensive ones.
Now wether the person is a sucker or not, in many circumstances the best price premium to be had from presentation come when someone is buying for someone other than themselves, and in that case, if they obtain what they feel is a suitable gift for an amount less than new, are they a sucker?