Best bet is your local Mom and Pop gun shop. We routinely do a dozen or so appraisals a week, don't charge for it, and have built a great fan base. We use Bluebook plus knowledge of what the market in guns is doing in our area. Gunbroker is good only if you realize that the prices set are often SWAGS and are good only if there are actual bids at those prices. Buy it now prices are often wildly inflated.
I often go to people's homes to do quick appraisals and rarely, if ever, make any offers. I do offer to put them in the shop on consignment. When we do an appraisal, we are honest in that we tell people that the book price is probably higher than what they'd get on the street. Also that what we'd give, should we think we can resell, is somewhat less than what they'd get selling themselves. We offer, when we think we can sell, 80% of what we're going to ask for a gun. Most shops offer about 55 percent.
Establishing the actual grade or percent of original condition is the hard part. I've evaluated guns that were described on the phone as "like new" and found them to be, at best, 80%. Find someone you can trust and go from there.