Can I be held liable for illegal activity for gun that I sold but lost the bill of sale?
The Gun Control Act of 1968 provides that persons “engaged in the business” of dealing in firearms must be licensed. Typcially however private sales by a person who “makes occasional sales, exchanges, or purchases of firearms for the enhancement of a personal collection or for a hobby, or who sells all or part of his personal collection of firearms." Only 9 jurisdictions and a small number of cities have regulation of private sales in regard to background checks, etc. No regluations are found in Colorado.
Therefore, it would be difficult for law enforcement to trace the gun from a private sale.
A bill of sale provides the buyer with a little assurance as to who the true owner of the weapon is and gives the specifics about the parties in the deal and the gun itself. Typically, the buyer retains the original copy of the signed bill of sale as proof the gun wasn't stolen from the seller.
The seller may retain a copy for his or her records but it is not necessary.