Kwai,
If you are serious about selling the plate no, just like selling any other things for that matter, your parents must decide how much is the minimum they are willing to let go. Then you have every right to ask for something higher to leave some room for negotiation. As soon as you got a serious offeror at a price acceptable to your parents, you can straight collect deposit and work on the details for transfer of the plate no.
The way you are going, asking for offers is a waste of everybody's time including yours. Say, I'm serious and offered you 20k for it. You wait for better offers. Other jokers offer 22k, 25k, 28k. You remained happy not knowing that you have lost a genuine potential buyer. By the time you realised it when you don't see the colour of the jokers' money, you've lost a good sale potential.
Another advice of mine is that you have to be realistic about the price. I come from Perak and I can tell you Perak plates are cheaper that KL or Selangor plate. Like someone mentioned earlier, other than 1 and 8, the other single digits are going for less than 10k. Once in a while, if more that one party like the particular alphabet issue, yes, they can fight to be more than 10k.
Okay, your's is an old plate. Can command a small premium from some. In my opinion, if you receive an offer in the region of 20k, you should consider it seriously and hopefully for you and your parents, the offeror is one with real money to pay.
Good luck!