As Mr. JeffG states, there just isn't much profit there, there is the liability issue and it also takes time away from paying customers, and there is at least one other issuee. Lots of folks see it as the first step towards registration. It works like this. I have a firearm that I want to sell to you. You want to buy it, so we agree to go to the local gun shop (LGS). The lgs has to accept it from me which means that he has to list it in what the ATF refers to as the "bound book," which is often just a series of form pages stapled together and not really "bound." That means he needs to see something with my name and address. All of that information goes on a page designated as "Acquisitions." So the LGS is now the legal owner of that firearm. In many cases the next step is that the buyer fills out a 4473 and the lgs either checks on line or makes an old fashioned phone call. The lgs is about to put the would-be buyers name in his bound book on the opposite page designated as "Dispositions," but surprise... the buyer is delayed or ineligible! Now the origianl owner has to fill out a 4473 to get his gun back so there is a paper (or computer) trail from that firearm to that person. Sound like a great idea? As of right now that information doesn't go any further, but if the little crumb snatchers running around Washington D.C. and lots of other places today have their way, it wouldn't take much to turn that data into a national database of gunowners and firearms.
The example cited above was just a routine sale. In some locations, city laws require the lgs to sit on all used firearms for 3 to 7 days while having the local police run the serial number through the fed database. Guess what happens when this check on a firearm that you've owned for several years reveals that it was stolen. You're out one firearm, the money you paid for it and you're now listed in official records as the reciepient of a stolen firearm. What happens in the locations where there is a waiting period and the guy who wanted to buy it decides that he really didn't want to buy it after the waiting period? Really not much of an upside to any of this.