I'm sorry and I would never defend Enterprise but I don't think you can sue someone for refusing to sell you something. It is bad business and unethical but the fundamentals are that even if you did receive a confirmation this does not constitute an enforceable contract, and as such it was simply an offer they revoked before you could accept (by paying) if they had taken money on the website this would be a different matter (which is why they don't) You Americans really need to stop suing at the drop of a hat (although we're getting that way to be fair)

Nice try though