it could be your "fault" the trade in page for their apple watch state one only! send in the watch case and not the charger and band and the likes and if one does one only get the case back
if the same hold true for the macs then it's on you,
this is from their page "Place the Mac mini secured with the foam pieces into the box. (Do not include extra items or accessories. If you include extra items in the box, we won't be able to return them to you.)"
from How to prepare your Mac mini for Apple Trade In - Apple Support
the cable is not a special apple cable it's a cable of the industrial std. c7 you should be able to get a new one dirt cheap in most supermarkets and AV stores
cable c7 - Google-