I think the OP initiated this mess.
Sure the buyers offer was a lowball, but look at the economy and the going prices on these cars. And like another person mentioned... his offer of $45k means he was probably willing to pay $48-50k. Negotiations are a dance between the buyer and seller.
The OP responded in a childish and arrogant tone and this set things off. If that's too low for you, just politely decline the offer.
And why the paranoia about giving out the VIN? Most online vehicle classifieds show it.