In some places "destruction" as defined by law can include several options, one of which is selling them for profit for the department, and another is furnishing officers. Another is of course actualy destroying them. "Destruction" under the law some places simply means they get a permanent new home not connected to the previous owner.
So it really depends on the department and what they choose to do.
So say someone commits a crime with a gun which is thiers. It will obviously not go back to them, but it also won't be eligable to be returned to a non prohibited family member. It is confiscated by the state, and once it no longer has value in the case it can be disposed of with a bit of discretion.
Obviously the local police don't do gun sales I am aware of to the public. However they can sell to an officer or an FFL and they can make thier way back to the public through those channels.
The 3 applicable penal code sections in CA are 12028, 12030, and 12071. 12071 is far too long to post and describes what an FFL etc is under the law and various restrictions they have. They often sell them for the department through an FFL and here is how that is worded in law:
12028. (a) The unlawful concealed carrying upon the person of any
explosive substance, other than fixed ammunition, dirk, or dagger, as
provided in Section 12020, the unlawful carrying of any handguns in
violation of Section 12025, and the unlawful possession or carrying
of any item in violation of Section 653k is a nuisance.
(b) (1) Except as provided in paragraph (2), a firearm of any
nature owned or possessed in violation of Section 12021, 12021.1, or
12101 of this code, or Chapter 3 (commencing with Section 8100) of
Division 5 of the Welfare and Institutions Code, or used in the
commission of any misdemeanor as provided in this code, any felony,
or an attempt to commit any misdemeanor as provided in this code or
any felony, is, upon a conviction of the defendant or upon a juvenile
court finding that an offense which would be a misdemeanor or felony
if committed by an adult was committed or attempted by the juvenile
with the use of a firearm, a nuisance. A finding that the defendant
was guilty of the offense but was insane at the time the offense was
committed is a conviction for the purposes of this section.
(2) A firearm is not a nuisance pursuant to this subdivision if
the firearm owner disposes of his or her firearm pursuant to
paragraph (2) of subdivision (d) of Section 12021.
(c) Any weapon described in subdivision (a), or, upon conviction
of the defendant or upon a juvenile court finding that an offense
which would be a misdemeanor or felony if committed by an adult was
committed or attempted by the juvenile with the use of a firearm, any
weapon described in subdivision (b) shall be surrendered to the
sheriff of a county or the chief of police or other head of a
municipal police department of any city or city and county or the
chief of police of any campus of the University of California or the
California State University or the Commissioner of the California
Highway Patrol. For purposes of this subdivision, the Commissioner
of the California Highway Patrol shall receive only weapons that were
confiscated by a member of the California Highway Patrol. The
officers to whom the weapons are surrendered, except upon the
certificate of a judge of a court of record, or of the district
attorney of the county, that the retention thereof is necessary or
proper to the ends of justice, may annually, between the 1st and 10th
days of July, in each year, offer the weapons, which the officers in
charge of them consider to have value with respect to sporting,
recreational, or collection purposes, for sale at public auction to
persons licensed pursuant to Section 12071 to engage in businesses
involving any weapon purchased. If any weapon has been stolen and is
thereafter recovered from the thief or his or her transferee, or is
used in a manner as to constitute a nuisance pursuant to subdivision
(a) or (b) without the prior knowledge of its lawful owner that it
would be so used, it shall not be so offered for sale but shall be
restored to the lawful owner, as soon as its use as evidence has been
served, upon his or her identification of the weapon and proof of
ownership, and after the law enforcement agency has complied with
Section 12021.3.
(d) If, under this section, a weapon is not of the type that can
be sold to the public, generally, or is not sold pursuant to
subdivision (c), the weapon, in the month of July, next succeeding,
or sooner, if necessary to conserve local resources including space
and utilization of personnel who maintain files and security of those
weapons, shall be destroyed so that it can no longer be used as such
a weapon except upon the certificate of a judge of a court of
record, or of the district attorney of the county, that the retention
of it is necessary or proper to the ends of justice.
(e) This section does not apply to any firearm in the possession
of the Department of Fish and Game or which was used in the violation
of any provision of the Fish and Game Code or any regulation adopted
pursuant thereto, or which is forfeited pursuant to Section 5008.6
of the Public Resources Code.
(f) No stolen weapon shall be sold or destroyed pursuant to
subdivision (c) or (d) unless reasonable notice is given to its
lawful owner, if his or her identity and address can be reasonably
ascertained.
.