Licence plates can not be generally transferred to a new owner. There are a few extremely limited exceptions (family members, basically) but even then the options are very limited.
Someone "buying" a personalized plate from a stranger is never going to be able to use it, so it'd be something they could hang in their garage/shop/house or whatever, but not actually ever attach to a car. Plates actually belong to the crown and could even be seized if the buyer brings it to a Service Ontario and tries to register it to a vehicle after having said they purchased it, for that matter.
Anyhow, as for worrying about a vehicle you sold, there is a process to absolve yourself of any liability - simply take your bill of sale and the VIN to Service Ontario and there is a form you can fill out that removes the vehicle from your name. Until the new owner registers it in their own it's basically in limbo as an "unregistered vehicle".
I discovered this last summer (I post about it here somewhere) when I went to buy a new plate sticker for my motorcycle and she asked if I wanted a sticker for the 2003, or the 1983. The 1983 was a motorcycle I sold somewhere around 1996 but the new owner apparently never (to this day) transferred it. It was at the same time when out of curiousity I asked her to check and found various other vehicles/trailers I'd sold over the years still in my name as well - I filled out all the forms right there and then and had them removed. A horse trailer I sold 3 years ago was still in my name, a 1974 camper trailer I sold around 1995 the same, as well as a few other things.
