Frankly, Gunbroker sold tells me what something is worth. That said, people from the coasts drive prices up a bit. The south and west mountain states drive it down a bit. Overall, it is the USA price. Selling locally ftf typically adds 1-5%. That’s basically the value of seeing it. Also, there is $100 worth of fees, shipping and transfers in a Gunbroker deal that local ftf can price in, but the cash requirement limits some buyers, so that has to price adjust down about 2-3%.
The other thing about Gunbroker is the buyers and sellers can be very educated. If you are selling something, there may be details that make it a $200+ better or worse deal. I think most of the “prices are all over” comments are looking at asking prices or not knowing those details. For example, a no box s&w 19-3 with rubber grips in 6” may sell for $5-600. A s&w 19-3 snub nose with OEM wood grip, box & papers could sell for $1300 with 100 bids!
That said, I know I over paid by $50-100 on my SA Hi Power clone by being too aggressive when I really wanted one!