I agree, i would not look for this part used. The amount of labor involved to install it is significant. My friend had a Honda Hornet 900 that needed to have the entire electrical harness changed and the mechanic charged him $500 (including new parts). Mechanic advised he has to basically take the entire bike apart to run the wires, its a big job. Have to pull the gas tank.
If the used harness is not good its a tremendous risk to have to do all that labor all over again.
When you say swap the Grom to a CB300 what exactly are you attempting to accomplish? You mean to change the motor of Grom to the motor of a CB300? You may be better off to sell the Grom used and then buy a used CB300. The Grom is a more expensive bike than the CB300.
And then consider after doing all the work how difficult it will be to sell a Grom that has been customized so deeply? Unless you are a collector, customizer or hobbyist I dont think it makes financial sense to do it.
2016 CB300F - 6200 miles - 100% stock config