Where do you suggest I pack hardback copies, with carry on or check baggage?
A flight from the U.S. to China will afford you two checked 50 lb. bags limited to 62 inches adding L + W + D. To optimize volume, that comes out to a 20 x 20 x 20 cube. You can get a cardboard box at UPS, Office Depot, etc. so as not to waste any weight in the luggage itself. With books, you will certainly reach the weight limit before filling the box.
So the strategy would be to fill the box with lightweight things like clothes and fill your carry on first with books. Your carry on will have weight limits as well, but will likely not be weighed.
When approaching the check in, DO NOT take your carry on or personal bag. JUST your checked bags. If the attendant feels the need to weigh your carry on, you may have to check that and they charge BY POUND. It will get expensive to the point UPS would be cheaper. The probability of weighing your carry on at the gate is next to nothing.
If you plan to make any trips from now until then AND you are FOR SURE going, take some of the library then and leave with a friend / coworker. New York to China is a popular route so ask around, post on social media, here if anyone is making the trip.
As far as selling unused books, post here. Otherwise, use bookscouter.com. Type in the ISBN and they'll tell you the highest bidder.