Except that by selling you increase the hash rate that is outside your hands and make it harder for you to mine in the future.
Not sure if it has a noticeable effect, though, given the crazy exponential increase anyway.
so, should he mine with it for 2 months, not make ROI, and then trash it, cause then it's used equipment and has no resale value?
No, he should sell every single piece of Gen 1 as fast as possible for the highest prices possible to make the maximum profit possible off that equipment. Come Dec, no one will be buying Gen1, anymore. It makes no sense to hang on to it.