Normally I would not pay anymore than 450$ because any more than that I'd rather just shell out the extra 100 and have one I know for sure is brand new. Right now you could probably get more due to the panic buyers. Really depends on where you live too, I'm in the DFW metroplex and can pretty much find a deal on whatever Glock I want on Texas Gun Trader
Is the seller a retailer selling a new one? If so, the best price I can find online for a new G20 gen4 is $579.
Or is the seller a private citizen selling one he bought but didn't use? If this is the case, the gun is technically "used" even if it's unfired, and isn't worth new gun price. It's up to you to decide how much you'd be willing to pay for one like this.
I got mine for $525 but the store owner ordered four a year before and had sold one so he gave it to me that cheap if I agreed to buy 500rds of ammo which was still pretty cheap. I have found usually a mom and pop lgs is your best bet. The guy I go to undercuts buds usually by 5% and sells a ton of guns that way. If I'm buying new that us where I go.