Not once they have been registered to a vehicle...Direct from Service Canada...
‘Transfers and gift certificates
A personalized licence plate can be transferred only once to anyone before it is attached to a vehicle.
You can purchase a gift certificate for a personalised licence plate at a ServiceOntario centre. Gift certificates are not available for sample plates or replacement licence plates.
Visit a ServiceOntario centre”
https://www.ontario.ca/page/personalized-licence-plate
I have about a dozen that I no longer use and would love to sell them if they could be transferred.