If i were to make a fiberglass enclosure for subwoofers, and make it to the right spec of a certain sub, including the sub in sale, would it be better to properly break the sub in for good business, or sell the sub with it new in box?
actually there is no real "break in" period with a sub. If the correct amp and set up is used there is no chance of some one "melting it down" unless they are totally inexperenced in the fact and use an amp that has to high of an out put for the specified sub. If i were you i would just sell it new not mounted or anything. When purchased make sure you tell them about the recomendations for the sub you know the input that it can take.
If its a kid, they never break stuff in. 5 years in the biz proves this. They will not know if you've hooked it up to some pink noise for a few hours and you'll be doing him a favour.
Hell some companys charge for break in.
In either case if you don't run it give him the advise and tell him not to come cryin' if he melts it.