From my basic understanding, it's not so easy to lodge a caveat these days compared to last time as you need to have a registrable interest to successfully lodge a caveat.

In your case, it would be easier to remove the said caveat as the previous buyer dud not sign any SPA, transfer form etc. Hence, he has no Registrable Interest per say for the caveat to remain in place but things like refund, etc has to be given back to him, if applicable.

But you should obtain your own legal advice.