Presumably you are in Victoria? Unless the number plates are tranferrable, they aren't worth anything except about $10 each to number plate collectors (as opposed to someone who wants to register them on a vehicle).
Don't get too excited about the $3000 to $4000 asking prices. I can ask $100 000 for my FE and $3million for my house, but I'll never get it...
Lots of blokes list number plates at stupid prices on ebay and other websites. The only ones making any money out of this is Ebay- they get the listing fees.
I think you would be better off selling the car registered, with a RWC and that way, the original plates can stay with the car. After all, not a great number of fifty plus year old cars still driving around with the original rego plates still attached.