Assuming that all your adverts, like the one you have on here, say that the personalised reg is available "by negotiation", my advice would be to change that and say that the "valuable private plate is included in the price".
We have debated before just how valuable the plate is, and in my view it is not worth a great deal at all. More to the point, you might have to wait an awfully long time to sell it. Whereas selling it with the car makes your life a little easier, and people love a freebie.
Basically, it might make your car stand out a little, or at least make it look slightly better value.