For online shops, only use places that are known to you, or for which you can find multiple positive reviews in multiple places.
For auctions: If you are not on Gunbroker or Gunauction.com, where feedback can be checked and assessed, it can be range from easy to fairly difficult to figure out whether or not you're going to have a rough time. Hibid, auctionzip, etc., are not the auction companies, they are just the bidding platform. Each individual auction house is different, and some are dirty, inept, and/or unprepared to handle firearms properly. (Many hibid auctions also have hidden fees for firearms. I have seen a $450 rifle have as much as $250 in hidden processing, transfer, packing, and shipping fees - beyond the buyer's premium and auction fees.)
Any reputable seller will allow a 3-day non-firing inspection period from when it hits your FFL
Perhaps that is the expectation, but quite a few reputable, high-volume sellers on Gunbroker don't offer anything beyond the transfer. You may have 2, 3, or 5 days for "inspection" before the transfer. But after the transfer, they offer nothing.
To me, that doesn't count as an inspection. It is more of a cursory glance.
Most gun shops are too busy and too limited on counter space for me to detail strip a firearm for what I consider a full inspection. --Especially when all they're getting from me that day is a transfer fee.
That whole "before transferred" concept just grates me, anyway.
I have seen firearms arrive with internal parts missing that had to have been present when the item was photographed. But the recipient couldn't really find the parts missing until they got it home for a cleaning and deeper inspection. And, because the item was transferred, the seller would do nothing for the purchaser.
I have also seen many, many bolt action rifles arrive with stocks broken in half. The buyers, knowing it isn't the receiving FFL's fault, take the parts home, contact the seller, and get told to eat it. 'Inspection and return period ends when transferred.'
That being said;
I have had the majority of auction sellers work with me after the transfer, if I noticed a problem - be it a shotgun fore stock that was held on by magnets, sights that went missing during shipment, a rifle that arrived in the left hand of the mail carrier with a chunk of cardboard box (w/ shipping label) and a bag of parts in their right hand, or a rifle that arrived with 12" of barrel hanging out of the box, with 16-grit drag marks across the muzzle.
I have had them offer returns, replacement parts, partial refunds, and even full refunds.
Most sellers
will work with you, if a problem arises.
In two instances, they tried to get me to exchange the firearms for something of roughly equal value. But, of course, I wanted *that* firearm and had to either settle for a partial refund (rather than an unobtainable replacement part), or take the hit because my own stubbornness meant that the seller could not remedy the situation (my fault for being unwilling to move, not theirs for wanting a damaged firearm back in their hands for inspection/repair/disposal).