How should I prepare one (currently built) LEGO set before selling it?
I have bought several used sets and it's a mix - sometimes people send the entire set built, sometimes split into larger chunks, and sometimes broken apart into individual pieces. It all depends on the current state of the set (fully assembled, partly assembled) and on the dimensions and availability of the shipping supplies. In the US it is easier to break the set apart and fit it in one of the pre-priced USPS shipping boxes that give you a uniform rate.

As a buyer, I always break the sets I receive down to the individual items and then inventory the sets using the full set inventory. Time intensive but it assures me I get the full set, and that there are no short-cuts/substitutions etc.