This selling is on hold for now.
to be clear, at this point, they have missed their opportunity for you to return it to them at their own expense. now that you are essentially doing a chargeback, you should fully expect to get the unit for free.
congrats
if you already have a 240V unit (or one on the way), go ahead and sell this whenever you feel ready.
edit:
my analysis of the situation is this. shipping from us→cn is very expensive. the cost is quite asymmetric.
they gambled on you not doing a chargeback (or PP dispute in this case... note that in the remote chance the dispute fails, a chargeback should be your next move if you paid with CC). they lost the gamble, so they can Sugon Deez, lol
the cheapest option would have been for them to offer you a partial refund.
next cheapest option would have been to send you a new 240V unit and say keep the old one. i say this because they are only out the wholesale unit price of the new unit + cn→us shipping.
next cheapest would be incurring a chargeback since they're out the full retail price of the old unit (including cn→us shipping and merchant fees) + chargeback fee + hit to their reputation.