I think Dale M is essentially correct. Let me give more detail by quoting the Theft Act 1968:
1 Basic definition of theft (1) A person is guilty of theft if he
dishonestly appropriates property belonging to another with the
intention of permanently depriving the other of it; and “thief” and
“steal” shall be construed accordingly.
(2) It is immaterial whether the appropriation is made with a view
to gain, or is made for the thief’s own benefit.
(1) Any assumption by a person of the rights of an owner amounts to
an appropriation, and this includes, where he has come by the
property (innocently or not) without stealing it, any later assumption
of a right to it by keeping or dealing with it as owner.
(2) Where property or a right or interest in property is or purports
to be transferred for value to a person acting in good faith, no later
assumption by him of rights which he believed himself to be acquiring
shall, by reason of any defect in the transferor’s title, amount to
theft of the property.
You assume the rights of an owner by placing a notice offering it to other people. Therefore it is theft. I presume similar laws exist in other juristictions.