You'll have a hard time believing anyone who is in the business. Similarly, anyone who collects Parkers and 'might be interested' will also be hard to believe.

Also, anyone who has collected Parkers for more than 20 years will be likely inaccurate as their opinion of worth is based on what guns were worth 40 years ago. Kinda like how your Grandpa gets outraged at the price of a soda and reminds you it only cost a nickel when he was a kid.

The best thing you could do is look through recent high-quality gun auctions and see what comparable guns sold for.