I feel like the ownership and selling of it are two, related, but separate issues.
It was 2 years ago, and regardless of how you feel you worded it, you paid half of the Oculus for him. It doesn't seem like it was "for the family" - otherwise if anything, it would have been unfair to expect him to pay towards it with his birthday money but not his siblings. I don't think you can retrospectively claim its "half-family". It's his, like the motorbike was his brothers - regardless of who paid for it.
With the motorbike, it has been given to a sibling - maybe its not all that valuable, or maybe it's value wasn't really considered. Assuming it is worth something - what would your position be on DD selling it? Will she have free reign on the profits, or will she have to share them with her brother as it was "his".
Here I think is where the issue is - he wants to sell something that ultimately was given to him but has been shared with the family - and the family still use. On one hand, I think ownership or otherwise, you can say "no, you can't just sell your stuff" but well that's tricky - its not like he's selling random items he needs to fund a fun thing - he's selling essentially an old toy.
I don't think you should get in the position of siblings paying siblings - or buying him out - He doesn't "owe" you anything but I think you can veto the sale of something / expect some slack in having to pay for something to be given back to you/the family.
Following this dilemma, perhaps avoid paying towards it (in case he decides to sell it on at a later date), but that doesn't mean you have to give it up for sale. Perhaps you and the rest of the family can find some unwanted items - phrased as "DS12 doesn't want his Oculus any more, but as we still want to keep it - perhaps we can find a few things that we can sell to make up the difference - and any extra profits can get some new games/other thing"