So, an update on the battery situation.
Ordered a battery from another eBay seller, they contacted me afterwards saying the same thing - none in stock, but happy to offer a refund. Emailed back and said "yeah, refund please".
At around the same time the original seller emailed and said "Hey sup, battery is back in stock, we've sent out the battery for you". No word from the second seller, and no refund yet.
I'm really not sure if there's something dodgy/scammy going on here. Either way, it was all done through PayPal so it won't be too hard getting it sorted, should it come to that.