There is probably no collector that will take in 600 models in one shot and offer you anything like fair value. And there is little chance you will find someone with no collection that seeks to start one with a load that huge. That's my experience at least from years of observing this hobby.

The market for the entire collection would be a reseller who will pay pennies on the dollar - which is logical - since they are not themselves collectors but instead are in resale business in order to feed a family. They know how to market the models in lots or individually.

So, if you are not prepared to sell them yourself or have a family member do it, and you have been in contact with the good people at Replicarz and are not satisfied with their criteria, you might try local estate sale or auction houses.

Locally, we did have one huge collection that was bequeathed to a local church and the church managed the sale for their benefit - so that might be an idea if that's an opportunity you might consider.

I do wish you good luck with the sale. It's not easy to do with something so personal.