I have my public master ready. Would I need to supply the wallet address or is that really not an issue ?
Not if you already have the xpub,
And it's faster to bruteforce with the master public key than the address since it'll skip a few rounds of
RIPEMD160 and
SHA256 per try if it doesn't have to derive the child addresses.
unfortunate he asked for the Dat file. Which I will never be sending out again. I did this many years ago and it ended out all over.
Hmm, that's odd, it's indicated in his website that it's not mandatory to send it.
He might be suspecting something fishy in your story (
like inconsistencies or something), services like his just don't want to waste resources on fake for sale wallet files.
Try your best to explain to him that the wallet is yours with consistent and honest information.
In any case, just don't send it to anyone including him like you said.