The seller is responsible for their own listing and checking it
Whilst there are postage options set by ebay in the newer listing format, there is ALSO a 'select postage myself' option the seller should have used, the seller should have used this option if the amount ebay had was not enough
Sounds like the seller does not care about his buyers, so, if I were you I would do just as he asked. Phone ebay and tell them the seller is refusing to honour the sale because of the postage (his error)
I would also personally add him to my blocked list to prevent revenge buying
And keep any messages as well in case you need to open an INR case
Good luck
