I’ll just file away the lip at the top of the battery hub that the batteries latch on to. A nice big elastic band can hold all the batteries in place, or if being picky, three individual elastic bands, one per battery.
Just my personal opinion, but I would not buy a used drone from a person I did not know and that was not done face-to-face. Also, I think the price is a bit high. Again, just my personal opinion.