No, according to JKR he bought the sword. But goblins believe that once the buyer dies, that it should be returned to the maker.
Goblins consider the true owner of an object to be the creator, not the purchaser. To purchase a goblin made artifact from a goblin is, in truth, to rent it for life. After the purchaser dies, by goblin tradition, the objects are to be returned to the makers. Because Godric did not see to it that the sword was returned to Ragnuk the First (through a will, or whatnot), they consider him responsible for the theft of the sword.