Gunbroker is a great site.

If a seller won't provide a 100% return within a short timeframe after receiving the firearm on your end, walk away.

If they won't provide good/better pictures of critical areas of the firearm, walk away.

If they make claims and won't back them up (or can't because the evidence doesn't exist), walk away OR only bid on what's there. Better to bid and win at a reasonable price and have it turn out to be the gun Elvis used to shoot TVs, than assume it was based on the seller's description and have it turn out to have been shipped to France in 1985.

Set a top and stick to it. Other similar guns will come along. Let others overpay. I've bid on a few dozen guns and won about 10 of them, which I think is a good ratio if you're sticking to your, ah, guns.

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Psalm 27:2