When a thing which is commonly the subject of sale is lost, if the owner cannot with reasonable diligence be found, or if he refuses, upon demand, to pay the lawful charges of the finder, the finder may sell it
(1) when the thing is in danger of perishing or of losing the greater part of its value, or
(2) when the lawful charges of the finder, in respect of the thing found, amount to two-thirds of its value.