Here are two real reasons that haven't been mentioned. You all probably won't like the first one very much.
1. Some of your guns may actually have been stolen, many many years ago, without your knowledge. If so, the original owner is probably long dead. Do you want to stir that pot?
2. Some of your guns may be the object of a latent civil dispute. For example, during the process of separation, wife sells one of husband's guns to her brother, and another to a local pawnbroker. The guns are pre-marital property and therefore not hers to sell, but not "illegal" for her to sell. The sales are not discovered until after all property has been separated and the divorce finalized. Husband discovers the guns missing and calls police. Police record the serial numbers and take a report, though because they deem it a civil matter, they do not investigate or recover the firearms. Three years go by. You buy one of the rifles, and proudly post a picture on the internet with the serial number. A private investigator tracing several pieces of property for the husband finds your post a year later. Husband sues for replevin. Now, you've got a lawsuit on your hands, which you will probably lose along with the rifle. Do you really want to stir THAT pot? By the way, this sort of thing actually happens.