If you found it odd that the goblins, specifically Griphook, resented the fact that Gryffindor's sword and Aunt Muriel's tiara had not been returned to them, then consider the case of the attitude to property in the island countries of the Pacific, particularly Fiji. I happen to live in Fiji and have learnt a reasonable amount about its culture and heritage over a decade or so here. Not that much admittedly, but compare me to Bill and the goblins and you'll have the general picture.
There are many artifacts that change hands for money and later get returned to their so-called rightful owners. It happens down here quite regularly with whale's teeth, something that a good number of you might find odd, as I did before living out here for quite a few years. Basically what happens is that at one time or another, quite typically in a ceremony, a whale's tooth (or tabua in Fijian), is handed over or bought by a visitor in an untraditional manner. The traditional way of handing over tabuas is hardly worth getting into as it doesn't relate to anything in the HP series whatsoever.
A tabua would never be buried with anyone, it would either rot or it would be passed down through the generations of the original owner's family. It actually is the tooth of a whale, not some carved or manufactured item. They have a very high value for the indigenous peoples of the Pacific, and old ones are particularly highly valued.
Anyway, once a tabua is discovered in the not right hands, despite its often having been bought for cash or other exchange, the original owners get right on to getting it back. There is in the ownership of tabua no way for it to change ever, even when traditionally exchanged; even then the tabua would be considered to be only on loan until the donee died. That is the theory, the practice occasionally differs a little in that a tabua can be passed on to another donee by the first donee and so on. The ownership never changes, at least that is the understanding I have gleaned from having attended innumerable traditional ceremonies while in Fiji and seen many tabua being presented at such ceremonies.
A little would depend on to whom a tabua were sold or exchanged. The indigenous people know that they are only receiving the tabua temporarily. If it were a tourist then said tourist would not normally be informed of the fact that the owner would consider that the tabua should be returned later. There have been instances, one not so very long ago, whereby tabuas have turned up in museums in countries outside the Pacific. Six tabuas were discovered in the UK somewhere, having been bought or possibly removed many years ago. The traditional owners recognised them at once, and don't ask me how because they look much the same to me. There is a kind of rope attached to the whale's tooth and that is probably unique to the family, tribe or individual original owner is my best guess on that aspect.
Once these six tabuas were recognised as belonging to a particular clan this clan notified the museum where they had been and the same were actually returned after some diplomatic manoeuvring.
The above example of recovery of tabuas is not unusual, often a tabua will eventually be recovered by the original owners and there is certainly no question of any money that might have been paid being refunded.
Of course I recognise that this is quite different from the Gryffindor's sword matter. Gryffindor did buy it, there's little doubt of that. However, the basic analogy is a good one. Once a tabua has been sold, and they can be, the person buying has no right of transfer upon death according to the traditional or original owner. There's the way in which the goblins, in my reading, consider the item, specifically the sword, but also Aunt Muriel's tiara, to always belong to them notwithstanding who currently holds it. Perhaps a form of trust, to give it a legal construct, would be another way of considering the goblin attitude. They are effectively saying: "I'll sell it to you, but you hold it on trust for me to be returned". The problem, naturally, being that they do not make this clear to their customer, and they certainly should rather than brood over something that they have little right to brood over precisely because they have probably not made it clear when selling such items outside their race that this is their attitude. It wouldn't be too good for business either I would think if the goblins' attitude to ownership became widely known to the human wizards with whom they deal.
In its simplest terms and putting all this into a goblin perspective - and there is a recognisable difference, in that there is no explanation as to whether an artifact considered to belong to goblins by goblins belongs to an individual goblin or to a group of them or indeed even to the race of goblins - your basic goblin, let's call him Griphook, as JKR did, says that a certain item belongs to a goblin because it was made by goblins. There is no indication as to whether the goblin attitude to goblin made items has been widely made known to human wizards. If it has then there would be some culpability on the part of the human taking over the lease (from a goblin perspective) of the item in question. If not then the goblins really only have themselves to blame.
The matter of tabuas and the goblin attitude to ownership of goblin made items are quite similar, I believe you may agree. IIrc in LotR the dwarves have a similar attitude.
On the whole it seems illogical, however, the explanation above would make perfect sense to a Fijian, and, of course, to a goblin.
Whether it is right is another matter, again it would be right to a goblin. I merely offer the example of the whale's tooth as a way to see the goblin point of view in respect of ownership. At least they have the consolation of keeping some, if not most, of the goblin made items in the vaults at Gringotts even if those vaults are leased by wizards and witches. The goblins probably see it as the vaults belonging to them and also whatever's inside. Whichever witch or wizard thought of letting the goblins run Gringotts may well have had this in mind when they did so. It would appease the goblin sensibility a little, or it would in my opinion.
Goddlefrood, who was rather surprised to get an opportunity to say something about tabuas.
Note: Due to a quirk in the way Fijian was transcribed by the first Europeans to do so, tabua is pronounced tambua. It would only ever be written as tabua, however.